While @SF's answer is great in detailing how re-entry heating works, it doesn't directly address the question to the root causes, thus I have taken the time to answer the question you clarified in the comments:
Why do we need such a high re-entry velocity?
Orbital velocity in Low Earth Orbit (LEO) is ~7.8km/s. In order to re-enter, a short burn is performed at apogee to lower the perigee deep enough into the atmosphere, usually in the amount of a few dozen m/s up to a few hundred m/s. Still, the re-entry velocity will be higher than 7km/s. The Apollo capsules had more than 11km/s of velocity upon beginning of re-entry.
You can not slow down further, because of the so-called "Tyranny of the rocket equation". In order to slow down from 7.8 km/s to 0 m/s, you need a rocket that is roughly the same size as the rocket that launched you into orbit. And in order to launch that rocket into space you would need a rocket that is exponentially bigger.
Even if you could, having an object drop dead from 400km height isn't desirable. If you came to a full stop at ISS altitude somehow, you would be going ~2500m/s once passing the Karmann line, and ~3000m/s once at 50km height. but instead of having a rather long path through the atmosphere that slows the vessel down to terminal velocity, it would take the shortest possible path, and will likely not be able to deploy its chutes to slow down, resulting in lithobraking.
During SpaceX launches, the first stage reaches space and returns to earth without a heat shield. This is because the first stage does not achieve orbit, it stays on a suborbital trajectory and is much slower (from a quick web search, it seems to go about 2km/s at seperation), and thus re-entry heating is not a major concern. Add to that the fact that it is a powered landing, going down on quite a steep path is not that big of a problem - you don't need to be able to deploy chutes.
Finally re-entry heating is better than alternatives. When Apollo returned from the moon, they re-entered with more than 11km/s. This velocity could have been reduced by circularizing in LEO first, then re-entering. But the problem is again with orbital mechanics: You need the same amount of fuel for the trans-lunar-injection than you would need to circularize back around earth when coming back from the moon. This is quite a big amount of fuel, and not taking that fuel but instead a bigger heat shield is by far easier (again, tyranny of the rocket equation).
Returning interplanetary (e.g. from mars) will again have much higher re-entry speeds, and circularizing around earth won't be an option, either. You'd need as much delta-v to circularize around as you needed to leave earth to get to mars. It is simply not feasible to take such amounts of fuel on that journey.
So, you need to have high velocities during re-entry. And with those velocities, SF's answer sums up quite good what happens.