Not all ads are crated - and placed - equal
Ads may be effective, or may be just a nuisance people utterly ignore. Online ads are mostly in the latter category, albeit FB & Google marketing are very able to convince people they are fully in the former.
Probably very targeted ads to the most gullible people have a higher return - but overall, they don't. Even people who don't use an ad blocker learn to ignore ads and find contents on the page - which in turn led to even more invasive ads which obtain the wrong result - people hate them.
Just look at how site news struggle to make revenues from ads, and have to add more and more of them, and a lot of click baits in attempt to make some money. In a magazine, you may give a look to a well laid out ad for a product you could be interested in, especially if it also has some useful data in (features, prices, etc.). But puts lots of bad ones on every right page, and you'll lose the reader interest quickly. Same for the web - clutter a page with many different flashy ads, and you've lost the reader. But they are interested in just selling lots of ads, not at making the effective, which would be much more difficult and expensive.
Especially since it's very difficult for advertising to measure the effects of their ads, since the ads platforms controls both delivery and their metrics - quite impossible for third parties have independent measures. So, basically, advertiser have to trust them - and nobody is ready to call "the emperor has no clothes!" for fear of looking stupid.