I suspect the problem is that the most expensive to serve users go past the initial set.
Yet I believe most of those users are the lowest value users - many of them will be bots and web scrapers, whilst others might be real humans doing long tail queries that have no relevant ads to serve.
I'd pay extra both to serve and to receive incredibly niche ads that would only show up for long tail queries.
That was the original promise of all the ad tracking, wasn't it? Outrageously accurate ad targeting? Reach the weirdos in your tribe wherever they are?
Now it just feels like drowning in a sea of things a bunch of giant corporations have decided everyone ought to want, and have been programming people to want en masse, rather than helping people find the unique, interesting little things they would actually want were it not for all the programming.
Yet I believe most of those users are the lowest value users - many of them will be bots and web scrapers, whilst others might be real humans doing long tail queries that have no relevant ads to serve.