Heat shields are only good for braking, not for acceleration.
Remember that the key to being in orbit isn't to be high up but to go sideways really fast. Launching a spacecraft means accelerating it all the way to orbital speed. Conversely, reentering and landing means shedding all that speed again.
It's fairly obvious that you need some means of propulsion to gain orbital speed. Of course you could also use propulsion to decelerate and land as well, but due to the way the rocket equation works, carrying all the fuel is extremely expensive.
But if you're landing on a planet with an atmosphere, there's a great alternative to propulsive landings. You can just let the atmosphere do all the work for you for free. Of course, all that kinetic energy needs to go somewhere, so it gets turned to heat. (As the second law of thermodynamics says, it's trivial to convert any kind of energy into heat, but hard to convert heat into anything else.)
So the reason you need a heatshield for reentry is that you chose to generate a lot of heat and need to shield the spacecraft from it. On launch, rockets fly the pointy end first, minimizing drag and heating, while on entry, they fly the other way around to make the atmospheric braking efficient. If you just reentered pointy end first, you'd shoot right through without much heating and then crash into the surface.
That said, drag and heating during launch is still an important concern, which is why streamlined fairings are used to protect the payloads from the airflow and heat.