I did a mock booking on Expedia (post #20 ) Before entering payment it has a list of amenities that are listed as NOT AVAILABLE and under that is seat selection. Yes the UA site is more in your face but UA cannot control what the mediocre OTAs do with their website.

Their used to be a job known as a travel agent. They still exist I am told and will do airline bookings for a fee. But once a person goes into do it yourself mode then it is up to them to do the work and research of the professional they decide not to hire. My late father could not book tickets on his own (macular degeneration, computer screens even with all the settings for impaired vision were impossible for him to see properly ) so he asked for help from me or paid someone to book the tickets.

I will hate on Basic Economy fares but still think people should look at what they are purchasing as consider whether it is right for them. I