Why are there so many different humans yet chimpanzees are just chimpanzees?
For fossils the answer is simple chimps live in forests and forest are really bad at preserving fossils, we lived in savannah which include some floodplains which are really good at preserving fossils. Thus we find a lot more human fossils than chimp fossils.

Otherwise Chimps exhibit far more genetic diversity than humans. humans are just a weird branch of chimp.consider the genetic diversity below. If we were not so  morphologically biased we sould easily conclude all hominids as just one species based on genetics.Chimps actually have quite a wide morphological variation its just we as humans are far better at noticing human variation.