People just say that to illustrate the fact that all relative truth is totally subjective and nothing more.
For example "you believe in God, therefore I shall believe in a Flat Earth because both are ridiculous".
My grandmother went to her grave insisting the moon landing was filmed in a Hollywood studio.
When I said that 'science proves it', she said 'can
you prove it without science?' and of course, I couldn't do that personally.
So, although I may disagree with those 'flat earth' people, I can see where they are coming from - not in as much as 'the earth is flat' but "you believe what you will and I shall believe what I will, no matter
how silly or improbable it may sound".
Like Baile said before, they are rebels...rebelling for the sake of a rebellion which does not exist, but
does prove a moot point about human perception, no matter how flawed it is.
P.S. The way I know the earth is a globe is through parallax differential - something that old cameras had to take into account whilst filming the horizon.