I wonder how much of that is innate and how much of that is because we're not used to fake pictures on a scale AI will allow yet
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.
It's not the abstract knowledge that something is possible that influences our gut reactions, its our experiences. When your friend group chat is chock-full of little jokingly-made fake images of each other, then your behavior of reacting to realistic photos as though they were real will dull and eventually go away entirely.