I think it's timing. You probably could have said the same thing about Google before they launched Adsense.
There's nothing stopping Facebook from building a platform to run ads across the web. My guess is they've discovered the web doesn't yet have enough social context to sustain the kind of ads they want to provide. (Instead, they end up feeling like creepy hyper-personalized banner ads.)
Because FB.com is about exclusively people (and brands to a degree) it's a suitable environment to show ads or sponsored content based on social behavior. FB is the social fabric for the web, but people still feel it all lives on their website.
Quickest way to change this might be to try to own the experience enclosure. (Build a browser, mobile OS, etc.)
Personal opinion is facebook has definitely over the last couple years realized they have monetize facebook.com itself to a large degree. Probably fb execs still as far as long term strategy view the 'platform' is king and not the website but it appears the length of the 'long term' has significantly stretched, and they realize they need to focus on monetizing the website for the next few years at least.
This is all conjecture on my part, no facts to back it up, but just pointing out its common for a business to 'think' XYZ is the monetization solution, and then after time and effort come to realize they have to pivot to another direction.
Fundamentally I think a lot of these 'platform' offerings have come to realize they cannot actually monetize the platform at the scale they would like, and have as a result made the business decision to monetize their 'application'/'website'. Twitter, google, facebook, etc this is becoming quite a common phenomenon. In fact the most stable platforms appear to either be outside of the direct control of a single for profit entity, or else an 'add on' feature of a entity highly profitable from another vector. If apple was dependent on the app store for the majority of their profit, I think we would see them having to do an about face, instead the availability of the app store is a 'feature' but their profits come elsewhere so they have freedom to let it operate as a platform.
I think it's timing. You probably could have said the same thing about Google before they launched Adsense.
There's nothing stopping Facebook from building a platform to run ads across the web. My guess is they've discovered the web doesn't yet have enough social context to sustain the kind of ads they want to provide. (Instead, they end up feeling like creepy hyper-personalized banner ads.)
Because FB.com is about exclusively people (and brands to a degree) it's a suitable environment to show ads or sponsored content based on social behavior. FB is the social fabric for the web, but people still feel it all lives on their website.
Quickest way to change this might be to try to own the experience enclosure. (Build a browser, mobile OS, etc.)