Because America is a deeply religious country, and that attitude is a necessity for taking religion seriously in the modern work.
People complain about him but Richard Dawkins had a point when he said that one of the reasons religion is inherently destructive is because the habits of thought needed to support it cause bad judgement everywhere else too. A society that admires faith is by definition a society that admires not caring if something is true. That person on Facebook *has faith *that vaccines cause autism, and she’s almost certainly been raised her whole life to think of faith as good.
For that matter, much the same way so many of Trump’s followers have faith that he’s going to do whatever they fantasize that he’s going to do. Despite the lack of any evidence whatsoever.