I’d say that from a user standpoint ads that are based on the page/site content are ideal, but from a business standpoint the drop in revenue (comparatively) by moving to such a model at scale makes it a non-starter (just a guess, no data to back it up).

More speculation: I wouldnt think this drop in revenue is due to quality of the ads themselves but rather how it limits the scope - if we’re no longer serving ads to the individual across websites then this makes the number of times their profile is sold/profited off of fewer.

Contextualized advertising gets my vote, though I’m sure there are other alternatives.