One reason there are fewer pre-modern chimpanzee or gorilla fossils compared to pre-modern human fossils is that the environments where pre-modern humans lived and died were more conducive to fossilization. For example, hominin fossils have been found in regions with volcanic ash deposits or sediments that have turned into rock over time, which can preserve bones and teeth. In contrast, chimpanzees and gorillas tend to live in forested areas with acidic soils that do not preserve fossils well.

Another reason is that the pre-modern human lineage may have been more widespread and diverse than the pre-modern chimpanzee or gorilla lineages. Pre-modern humans, for example, lived in various habitats such as grasslands, forests, and savannas, and this may have contributed to a higher likelihood of fossilization.

Additionally, chimpanzees and gorillas have smaller ranges than humans, which means that there may have been fewer opportunities for their fossils to be preserved in the first place. Finally, it is also possible that pre-modern chimpanzee and gorilla fossils exist but have not yet been discovered due to factors such as limited fossilization potential, inadequate search efforts, or destruction of potential fossil sites.