Because "they" (the government, academia, THE JEWS, aliens, all of the above) want to keep humanity ignorant about the true nature of God (a lot of flat earth conspiracy theories overlap with fringe Christian belief,) the universe and human potential in order to maintain totalitarian control over reality.

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