Why do some wacky theories such as the flat-earth theory gain so much popularity?

Not everyone's brain works well. Ever otherwise normal people have stupid ideas. The problem comes when they refuse to update and discard trap.

Idealists, be it religion or alt-right are the worst about it.

It’s like insisting there is a Santa Claus. It doesn’t really harm anyone but it doesn’t really help either.

Tide Pods, same question.