The first and only reason there's no transitionary species between humans and chimpanzees is that we simply are not descendants from them, instead we share a common ancestor with the apes (which was probably more chimp-like if you want to envision what it was). We both evolved from the common ancestor because we fit different environments better. Eventually we developed sufficient intelligence, stemming from some environmental pressure chimps did not have, to manage survival in almost any environment, leading to us leaving them "behind".

From our common ancestor, there is a lot of continuum of hominids in the fossil record however. At some point humans and neanderthals did coexist as a spectrum of hominids coexisting at the same time, and we did "feed back" the most-fit DNA by interbreeding with them until we bred them out of existence.