I think most people are just repeating what they learned as children and haven't given it much more thought. So the answer is usually some combination of religious influences, when you were schooled, and what textbook your teacher was using.

In 2008, this is what a certain group in Hollywood thought 10,000 B.C. was like... anachronisms and all.[0]

[0] https://en.wikipedia.org/wiki/10,000_BC_(film)