I don’t think advertisers want to spend money for bot clicks.

That's why the social networks don't want to talk about the bots, but they are happy to have them.

Or maybe they do, because the big open secret in advertising is that they do spend quite a bit of money on exactly that

I don't even think it's a secret anymore, open or otherwise. As long as Vanity Metric Goes Up, it doesn't seem like anyone, anywhere, actually cares how much "economic activity" is fake.

I suspect it’s the “nobody got fired for buying IBM” mentality.

Who in marketing doesn’t want to champion the success of “we got 25% more views this month!”