Most of it is innate. We've known photos can be faked for a long time, it was a lesson in my history textbook in high school. We've known that anyone could fake a photo using a computer for a long time too, image editing software got good in the 1990s. And so on.

Rationally, we know all this. But biologically, we're wired to react first and think later; our reactions often shape our behavior more than logic. That's one reason that advertising and propaganda work; repetition trumps rationality. Add in the fact that large numbers of people are gullible fools who can be trained to uncritically repeat and spread untruths that appeal to confirmation bias.