If you look at all the flat earthers, the problem that eventually sold each of them that the earth was flat, and started their journey down the rabbit hole, is no matter how hard they tried, they couldn't prove to themselves the earth curves and measure that curvature.
No, it really isn't.
There are plenty of ways they can prove to themselves Earth can curve.
If they put in a tiny bit of effort they would be able to.
They have a much deeper need for Earth to be flat.
It could be that they accept their religion indicates Earth is flat and they don't want their religion to be wrong.
It could be that they were conned into it and don't want to admit it.
It could be that they are paranoid and are rejecting what they have been taught.
It could be that they simply can't handle reality and need to escape into a fantasy.
It certainly isn't because they can't prove to themselves Earth curves or they don't know how to measure it.
This is obvious from how many things that have been provided to them which would allow them to do that which they just reject.