Probably a combination of scale and ease. The people on the internet who try this gambit have to actually write each of their posts to their target audience which acts as a time/money barrier which is now gone. But the bigger one is that inventing these identities convincingly is hard. There's a million little shibboleths, expressions, in-group references that you have to know to not out yourself and if you get right imperceptibly make your argument much stronger. To pick an example that is to me both funny and malicious, the right wing middle aged white suburban men who sometimes get caught pretending to be gay black disabled veterans in internet political arguments have a very real lived experience gap that they have to navigate. But AI is scary good at that kind of fuzzy messy logic and can bridge that gap.