- 500 years ago people knew that earth was flat,

Actually, 500 years ago, almost everyone knew that Earth was round.
The story of Columbus you were taught in grade school is actually a
fantasy dreamed up by Washington Irving, possibly influenced by
Antoine-Jean Letronne. God knows why this Lie is still being taught.

Because it feeds modern people's sense of superiority -- "chronological
snobbery" as C. S. Lewis called it.

One of the myths of the modern worldview is that nobody had any brains until
just recently.