Basically it's a problem of interface. The airlines see no need to create an entire method of communication between the web and their computers - only enough that the majority of consumers are satisfied, which is what AP has done. If you are one of those who isn't (satisfied), then you go to book on UA. Otherwise you pick up the phone - trust me, I am not particularly happy when I have to do it too, because many agents are clueless, but sometimes you are lucky.