Christianity is false, but it is harder to falsify it then it is to show that Barrack Obama is not a non-sapient extinct mammal. I can prove the second false to a five-year-old of average intelligence by showing a picture of Obama and an artist’s rendition of a mammoth. It would take some time to explain to the same five-year-old child why Christianity does not make sense as a description of the world.
This difference—that while both claims are false, one claim is much more obviously false than the other—explains why Christianity has many adherents but the Obama-Mammoth hypothesis does not. And we can usually infer from the fact that many people believe a proposition that it is not transparently false, making it more reasonable to investigate a bit before rejecting it.
One can think that individuals can profit from being more rational, while also thinking that improving our social epistemic systems or participating in them actively will do more to increase our welfare than focusing on increasing individual rationality.