Sometimes you don't want to rebuild an index, no matter what its fragmentation.

Sometimes the fragmentation doesn't matter.

The challenge of deciding, automatically, when one can take a huge table completely offline for long periods is extremely hard, there's no way they'd get it right every time, probably not even most of the time. Hence safer to let the people who actually know the system and its usage patters decide.

Edit: Since the OP decided to edit his question out, for anyone interested the original question was asking why SQL doesn't include automatic index rebuilds.