Why haven’t we landed probes on Neptune, Jupiter, Saturn and Mercury with onboard cameras? Voyager 1 and Voyager 2 were sent out in the 70’s, with 70’s technology, is there technology available that could facilitate things like the mars rover landing on these planets?
First of all, landing on one of the gas giants is VERY difficult. They don't really have a surface, so what does landing even mean for such planets? That being said, we actually have sent a camera to both Jupiter and Saturn, although the spacecraft did not survive to tell the tale in both instances.
For Jupiter, we sent first the Galileo Atmospheric Probe to study Jupiter up close. It collected information about the atmosphere, radiation environment, etc. No camera was included because there wasn't the bandwidth, the mission had a limited duration, and in any case, it probably wouldn't have seen much there anyways, much as you don't see much inside of a cloud. Galileo eventually also entered Jupiter at the end of its mission. So far as I can tell, this is its last picture ever taken.
Cassini did the same thing with Saturn, but no atmospheric probe there. This is its last image.
As for why nothing to Mercury, it's a little known fact that it's actually harder to get to Mercury than it is to leave the solar system. In addition, Mercury isn't as interesting as some of the other planets. There have been a few plans for landers over the years, but nothing has happened yet. We will get one there eventually. To land on Mercury requires a delta-v of about 12 km/s if launched directly from Earth. To land on, say, Europa, the moon of Jupiter, it takes roughly the same. In addition, one has to deal with the high heat and other challenging things. It can be done, but is very challenging. If you want to know more, you can take a look the proposed BepiColombo lander.
I'll genericize the question to "why haven't we done X in space?"
Every space exploration agency has a rather limited budget, and the planetary exploration divisions within those space exploration agencies have even more limited budgets. NASA's budget, for example, is less than half a percent of the US federal budget, and NASA's planetary exploration budget is less than a tenth of NASA's overall budget. Other governmental space agencies also face restricted budgets.
This means that space agencies have to make a trade off between their limited budgets and what they want to do. What they want to do is in turn a trade off between the scientists who make competing proposals for the space agencies' limited funds, the political organizations that fund the space agencies, and the general public who elect those politicians.
Missions that study the Earth, the Moon, Mars, and the Sun receive the lion's share of those already limited planetary exploration funds. Missions to the outer planets, their moons, the asteroids, or to Venus and Mercury compete for a very limited budget. A mission to one of the ice giants would inevitably be a Discover class mission (multiple billions of dollars). The current interest is in moons that might support life. There is no room in any space agency's budget for a Discovery class mission to Uranus or Neptune in the foreseeably future. Mercury is surprisingly expensive to visit; there is no room in those budgets for a Mercury lander, either.
The surface of Mercury is extremely challenging for a thermal designer. A single day/night cycle is 176 Earth days, so except in polar areas permanently shadowed, a lander will experience long periods of extreme heat and cold. In the permanent shadows, it's cold and there's little light for pictures or solar power.
An orbiter has it much easier, as it can orient heat-tolerant solar panels and shields toward the Sun, and control, by design, the power flow into the body of the spacecraft. It can radiate the heat conducted and dissipated to deep space from its dark side.
The lower levels of gas giant atmospheres are even hotter and the pressures are beyond the limits of any known pressure vessel technology.