Not to put too fine a point on it, but (1) we did once[1], because you're right that it seems like an obvious fit. But in practice (2) it got absolutely crushed in the market performance-wise by more sophisticated algorithms like Facebook's and Google's (Amazon plays in this world too, though they have an easier space to search). It turns out that the fundamental game in the ad world isn't serving ads that site administrators and content creators think their users want to see, it's figuring out what the users of that content actually want and showing them that instead.

And indeed, that cuts the site operator out of the loop, and forces them (if they want to make money from these ad algorithms) to design a site that will attract users with easily-intuited advertising needs. And the linked article doesn't have that.

[1] And still do in parallel niche markets like porn.