You are talking two specific genera: Pan for the chimps and Gorilla. Both dwell in tropical forests, by and large, which is not conducive to fossilization. The John Hawks website that astro linked to has any number of fossil ape genera listed. (Note that many of these were “plains apes” and at least one lived in swamps – both better suited to fossilization than forest terrain.)
Some chimpanzee fossils have been found very recently. It seems one wandered out of the rainforest and lost some teeth, half a million or so years ago.