Most airlines leave certain percentage of seats to be assigned on the day of departure at the airport.

Also, on some airlines (e.g., TWA, not sure about UA) exit row has to be assigned only at the airport (even for elites). The trick here is that while it has to be assigned at the airport, it doesn't have to be on the day of departure. If you are at the airport a week (or a month) prior to departure, you can get exit row seat assigned.