I sure flat-earthers can give you a plethora of reasons, but if you want one of them, just watch this particular bit: https://youtu.be/Q7yvvq-9ytE?t=1514

Spoiler: According to her, it's money. They make a lot of money on that lie.

Of course, that raises a few more questions, but I don't think it's worth the follow-up.

Here's a little trivia for you I learnt on that video: we are ball-earthers. lol