Advertisers buy from ad companies, ad companies rely on advertisers believing it works.
Advertisers don’t sell ads, they buy them.
Advertisers don’t sell ads, they buy them.
Ad companies sell ads.
Advertisers purchase ads.
Users see ads.
That’s it.
Ask the company who uses lots of nails whether hammers work.
I mean maybe the purchaser employee at the company has some conflict of interest around the question -- if they're specialized in evaluating and negotiating hammer purchases they'll not want the company to switch to screws.
But the company as a whole seems to have no vested interest in promoting hammers. They presumably want to sell the stuff they make with hammers and may lie to you about how awesome those products are. But lie about hammers? Why?
Just because someone believes it works doesn't mean it does.