Why do asteroids burn up in the atmosphere?

Friction is a part, but only a part, of the reason asteroids usually "burn up" in the atmosphere.

When a meteoroid enters the upper atmosphere of Earth, it is travelling at a very high speed; in the neighborhood of 30,000 MPH. As it enters the atmosphere the air in front of it is compressed. When a gas is compressed, its temperature rises. This very hot air then heats the leading edge of the meteoroid as high as 3,000 degrees F (1,650 degrees C).

Then enough heat is generated that the meteoroid catches melts and eventually vaporizes or shatters because of shock effects. Most meteoroids that enter burn up completely, a small percentage make it to the ground and an even smaller percentage actually cause catastrophic disasters.

The meteoroids that make it to the ground, at least the small ones, are not extremely hot. Just think, once the object has been reduced to a few ounces or a few pounds, it is not going to heat up any further in the last few miles before impacting the Earth. It will most likely cool down. Only the very large ones will cause devastation and impact craters. and they will still most likely not be hot. How hot can an object of a few tons that was travelling through cold space get after a few minutes of heat caused by compressing the air it passes through? Like ice, only the surface melts. The interior will still be cold until its internal -250 degree C temperature is raised. That could take a very long time for a large object.