The tricky bit here is that advertisers don't need their ads to work, they just need to convince their customers they do. Even if there are reasons to question the metrics used to measure effectiveness (see e.g. [1]) this is all meaningless if ad companies still succeed in convincing people they are selling something of value. Which ironically does mean that one way or another advertising companies are good at advertising something it just might not be the thing they're getting paid for.

[1]: https://thecorrespondent.com/100/the-new-dot-com-bubble-is-h... [1, hn]: https://news.ycombinator.com/item?id=21465873