The unmanned objects cannot bring back new samples or identify elements unknown on Earth. With an abundance of helium-3, traces of liquid water yet to be explored, and its poles yet to be studied, why are they sending just unmanned probes?
The short answer is that humans are heavy. For the weight of one human you can haul a big bunch of scientific instruments. Not to mention saving the weight of all the food, water and air that a human would need to eat, drink and breathe during the trip. Humans get cranky when they get too hot or cold. A well-built space probe can deal with much larger swings of temperature/radiation/etc.
Also, you're totally wrong about bringing back new samples. There's nothing particularly stopping any of the missions from bringing back samples — even much larger samples if they don't need to lug said heavy human back as well. It's just they find they it's easier to do the science in-situ.
All that said, human exploration of space does have a certain appeal to it. Sadly, it tends to be really expensive and risky. Remember the uproar after the Challenger disaster? That's something NASA and other space agencies want to avoid.
The political goals of the Manned Moon Landing program were implicit in President Kennedy's "We choose to go to the moon" speech.
Taking a chunk ending with the infamous quote:
Those who came before us made certain that this country rode the first waves of the industrial revolutions, the first waves of modern invention, and the first wave of nuclear power, and this generation does not intend to founder in the backwash of the coming age of space. We mean to be a part of it--we mean to lead it. For the eyes of the world now look into space, to the moon and to the planets beyond, and we have vowed that we shall not see it governed by a hostile flag of conquest, but by a banner of freedom and peace. We have vowed that we shall not see space filled with weapons of mass destruction, but with instruments of knowledge and understanding.
Yet the vows of this Nation can only be fulfilled if we in this Nation are first, and, therefore, we intend to be first. In short, our leadership in science and in industry, our hopes for peace and security, our obligations to ourselves as well as others, all require us to make this effort, to solve these mysteries, to solve them for the good of all men, and to become the world's leading space-faring nation.
We set sail on this new sea because there is new knowledge to be gained, and new rights to be won, and they must be won and used for the progress of all people. For space science, like nuclear science and all technology, has no conscience of its own. Whether it will become a force for good or ill depends on man, and only if the United States occupies a position of pre-eminence can we help decide whether this new ocean will be a sea of peace or a new terrifying theater of war. I do not say the we should or will go unprotected against the hostile misuse of space any more than we go unprotected against the hostile use of land or sea, but I do say that space can be explored and mastered without feeding the fires of war, without repeating the mistakes that man has made in extending his writ around this globe of ours.
There is no strife, no prejudice, no national conflict in outer space as yet. Its hazards are hostile to us all. Its conquest deserves the best of all mankind, and its opportunity for peaceful cooperation many never come again. But why, some say, the moon? Why choose this as our goal? And they may well ask why climb the highest mountain? Why, 35 years ago, fly the Atlantic? Why does Rice play Texas?
We choose to go to the moon. We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win, and the others, too.
—John F. Kennedy ❶
The goals are clear:
- to ensure that no one else weaponizes space
- to put the US into the leadership role in space technology
- to give a common goal to the US people
- to provide new technologies to improve life in the US
The unspoken goals add:
- to make the USSR take notice that the US wasn't going to back off from confrontations with them.
- to establish clearly to world leaders that the US could deliver a warhead anywhere in the Earth-Moon orbital envelope.
- to fuel federal funding into new jobs created by the space program
- to distract people from the ongoing war in Vietnam
By the mid 1970's, NASA had been too successful at launches to the moon. Even Apollo 13 successfully returned. NASA made it look easy, which was good for geopolitical reasons, but not so good for continued funding. The military goals had been shown. The technology goal had resulted in a lot of spinoff technologies improving life in the military, and in civilian life.
Moon shots are expensive. The cost to the moon was measured in hundreds of millions of dollars per launch, aboard the singularly most expensive vehicle to date, and it was a one-use vehicle. Further, the actual science goals could be achieved in near orbit for around 10% of the costs of moon shots. Skylab was a reuse of already built hardware intended for Apollo 18 - science on a budget.
As for the USSR, the war in Vietnam was too big to not be noticed; it even crossed into the consciousness of elementary school children. (This author included.) The brinksmanship continued, and President Johnson had no personal commitment to continuing the space race, having already "won" it; President Nixon and later Carter likewise saw uses for most of NASA's budget planetside.
Only the actual science goals were left. And that didn't require lunar missions, only LEO missions. The post-lunar missions tended to be longer, and still cost less to launch, reducing further the budget draw.
Since then, there has been no compelling "race to the moon" and no compelling mineral exploitation opportunity. All of which adds up to "No compelling reason to spend that much on disposable rockets." And smaller, non-manned-mission lunar rockets with probes are, pound for pound, less expensive than the manned lunar missions.
The US government has been divesting itself of basic R&D and funding that sort of activity less and less. Initially it was all about beating the Russians to the moon and scoring a political victory. The science and technology was a benefit of that, but with the change in thrust by the US Government towards the military industrial complex, further investments weren't pursued. The thought was that commercial entities would pick up where the government left off. As of yet, the cost benefit analysis doesn't make these sort of ventures economically viable.
There are also arguments that we can accomplish the basic science with much less risk by using robotic probes instead of manned missions, at a much lower cost.
It is easy to send a probe instead of humans because humans need food, sufficient amount of air, living temperature (around 30 °C) and shelter, but probes only need electricity and they must be landed safely. They will do their work autonomously or controlled by a ground station.
It is possible to return a sample from the Moon without humans, and we can use a robot to drill a sample and place it in a reentry capsule and return that to Earth (example: Luna 16). There are many difficulties, but sending and returning a human is a lot more difficult. In addition, the humans must be trained.
It's all about cost and safety. If we lose a probe, it's a loss of cost. But if we lose a crew, it's a loss of both life and cost.
Short answer: there has been no good reason to justify the risk and expense.
When Kennedy delivered the speech which kicked off the effort culminating in the Apollo Moon landings, there was a geopolitical motive. Even during the Apollo program, there were doubts raised as to whether it could be justified. It was seen as very expensive; there were protests arguing that Apollo funding could be better spent feeding and sheltering the poor, despite the fact that the Apollo program represented a very small slice of the U.S. federal budget.
The Apollo program was all about demonstrating the capability of the American military-industrial complex in a non-threatening manner during the height of the cold war. It happened to provide some scientific benefits, and the subsequent technological spin-offs have been hugely beneficial, but these weren't considerations when funds were being approved.
The geopolitical motive at the time of Apollo no longer exists - it disappeared once successful lunar missions were shown to be readily repeatable (cost and risk notwithstanding). The only motivations for crewed lunar activity post-Apollo would be scientific and/or commercial. Landing humans on the Moon and returning them safely to the Earth remains about as hazardous and expensive now as it was during Apollo. Science can be done robotically at far less cost than sending a human, and without placing human life/health at risk. Successful commercial exploitation of the Moon would depend on it having useful resources which can be economically extracted and transported to the point of consumption. There is almost certainly no material resource on the Moon worth hauling back to Earth. There might be a case for mining the Moon to supply facilities positioned at one of the Earth-Moon Lagrange points, if such a thing were ever to exist. About the only other reason to land humans on the Moon would be to use the Moon as a proving ground in preparation for sending humans to Mars; the value of doing this has been debated.