In underregulated capitalism, the reason is very simple: because they get away with it. This is why they despise regulation; it enforces accountability. Want to advertise? Ensure the ads are legit. It doesn't work when you say 'we didn't know beforehand' when a significant percentage is scams. That is when you start to curate the content beforehand.

In any other normal market, the middleman would be held accountable. Big tech? Not so much. We seen the slap on the wrist regarding Microsoft at the start of the century/millennium. We can also witness the same with regards to the garbage coming from China, where China doesn't take responsibility. Learned from the best.