Flat Earthers hold their belief because they know something's off about the official narrative and it's the closest they've gotten to the truth. It's the most stigmatized and ridiculed theory, not because they believe in a flat earth, but rather the other truths many have uncovered along the way.
(media.communities.win)
Speculation
FE does not say planets are fake, but rather they are not planets but celestial light bodies revolving around the plane-t earth, you can watch a lot of footage of YT showing the planets as bodies of light, anyhow if you are interested this is a good place to start:
https://www.youtube.com/watch?v=WAeBPskvP4Y