Why do the stages of a rocket burn up in the atmosphere when they come back to earth?

Spent rocket boosters fall from space into Earth's atmosphere with enough speed to cause friction buildup, which in turn causes enough heat to literally burn up the booster segments. Though they look heavy, they're actually made of a composite aluminum with high shear strength, much as the same material aircraft wings are made of, or in some cases fiberglass. Regardless of material, they cannot survive the heat of re-entry into Earth's atmosphere.