Many agents are simply not trained on booking flights. Sounds dumb but it is true and it is not only UA, this is an issue with lots of airlines. By the time they figured out how to do it correctly, there would be a lot of angry people in line wondering why it was taking so long. Agents that used to work in reservations or CTO should be able to do it quite easily. Somebody that was hired in the last few years to stand by a kiosk and issue a bag tag every once in a while has saved the airlines money in training costs but do not necessarily make it convenient for somebody needing a booking.