Why haven't crocodiles and other reptiles evolved since the Paleozoic era?

They did evolve a lot, but it's true that there were already crocodile-looking animals a few hundred million years ago. All it means is that the basic crocodile body structure has been very successful and didn't need to be changed drastically. Evolution doesn't say that all species should evolve at the same speed or in the same way. If it ain't broke, don't fix it!