alt Hacker NewsRyanair (EU) also does this, but the US is indeed pretty obnoxious here.
United even has commercials before the safety video; combined with the "if you're watching explicit content on this flight, please mind the children" announcement, those flights onestly honestly felt pretty surreal to me.
United has gotten worse and worse with this. The ads after (not before) the safety video, and also before each movie you watch (and it's usually the same ads before every movie). A few years ago the ads were skippable, but not anymore.
The flight attendant also makes an announcement about the United-branded credit cards near the beginning of the flight.
But this is really just an illustration of what the top-poster of this thread said: flying people places doesn't make enough money, so they have to pursue other revenue streams.