It is impossible to prove to a Flat Earther that the Earth is a sphere, because they reject every single bit of evidence or logic that shows that the Earth is a sphere. Ultimately, they can't observe the spherical Earth with their own senses, therefore they don't believe in it. But they do believe, for example, in radio waves even though these can't be detected with their own senses, either.
And ultimately, the reason why there is no scientific proof of the FE comparable to the proofs of the RE is.... conspiracy.