Having worked on a design system previously I think most people, especially non-frontend developers, discount how hard something like that is to build. LLMs will build stuff that looks plausible but falls short in a bunch of ways (particularly accessibility). This is for the same reason that people generate div-soup, it looks correct on the surface.

EDIT: I suppose what I'm saying is that "The paid products Adam mentions are the pre-made components and templates, right? It seems like the bigger issue isn't reduced traffic but just that AI largely eliminates the need for such thing." is wrong. My hunch is that AI has the appearance of eliminating the need for such things.