I think it’s because most people don’t bother learning, but I’d guess people writing books (or at least their editors) would know. AI eats up all the books and learns how to use em dashes. The majority of the internet-using population does not use it. And so you get the heuristic that em dash = AI. This is just a total guess, by the way.

Looked up the difference between hyphens, em dashes, and en dashes in high school. Maybe for curiosity, maybe for some assignment, I forget by now. Started using em and en dashes, not going to stop now.