People in prehistoric times must have been the most skilled stone carvers ever. This was the technology of the era. What they might have lacked in tooling they surely had in craftmanship and available time.
Is it a case of not being able to carve something more realistic or not interested in doing do?
Maybe confronted with the later artistic fashion of emulating reality (started in Ancient Greece and ended in the early 20th century) they would retort:
Why waste your time reproducing something that already exists?