Because google adsense has already effectively captured that market.
Agreed that adsense serves ads to 'open web' properties already. However they do this with user tracking, just like facebook already does. The suggestion above is to instead use contextual ad targeting.
Secondly - adsnese has no access to facebook's walled garden hence facebook could exclusively served contextually relevant ads on its own highly trafficked sites free from competition.
Thirdly - what's wrong with competition? facebook has the scale, relationships with publishers and advertisers, and technology. They could, if they wanted to, take a page from Apple and compete on the concept of privacy if their brand wasn't so tarnished by their insistence that they surveil their users.
Back to the core point - facebook and others facilitate transfer revenue from high quality sites (railway hobbyist sites in this example) and encourage surveilling individuals for only facebook's benefit.
It’s kind of amazing to me that it’s now become ingrained that advertising must involve user tracking when even 15 years ago companies like Google did almost no tracking at all.
nit: AdSense does both contextual and personalized advertising. It launched as contextual only, and that is still part of what it's doing: https://en.wikipedia.org/wiki/Google_AdSense
(Disclosure: I work on ads at Google, speaking only for myself)