“Why does everyone believe ...... albeit several thousands of years ago?..”
a logical question.
but the answer is ; the fossil record does not support that theory.
unless, fossil record (age) is misunderstood...?
Relying solely on fossils can be misleading.
Assume that some dinos did actually survive up until at least the paleolithic era. They may have been hunted to extinction and eaten by early humans and their bones left to turn to dust. Even small animal bones that were eaten by humans or other animals are only preserved in caves or burial sites. Those left to the open, decay and are never seen since they returned to the soil.
Fossils are very old, but assuming that the animals all died out because of that is not logical..............