Partially just because it can't be done at their scale. With millions of people posting new videos every day, they can't possibly review them all manually, and AI (a misnomer if there ever was one) is incapable of understanding. I also think they think they're being "fair" by establishing simple, objective rules like "no guns" that apply to everyone equally (though they don't really, of course). They're probably also getting guidance from their lawyers about potential liability issues, but I don't have any insight into that. Overall it's shallowness and a focus on their own business interests more than anything else.