The motivation to pay is to generate better spam on the one side, and to spot that spam on the other. Plausibly, a lot of content will go through multiple iterations of both of those before reaching any actual human. Same game that's going on now, just cheaper (even more automated) and at higher volume.

In the US presidential election billions of dollars are spent each cycle. If you think advertisements are the only use for AI you're underestimating the scope of the problem.