Its mostly around the CPC business model for advertisers, and the performance-optimized block of ads for publishers. Pubs get steady payouts with a single block of code, and advertisers only have to pay for clicks, which are generally pretty cheap.

AdSense has a similar CPC model, but leaves it to the publisher to place individual ads, and suffers from bifurcated demand- much of the Google ad money flows to their CPM / Impression based Ad Exchange banner ads.

Direct response advertisers do very well with content-rec style ads. I would never think its okay to run them as a publisher, but thousands of big sites can't say no.