Why UA doesn't buy buy AZ?

Question

Great Loop, Don't you think so?

Answer 1

Not really.
Look how much trouble UAL had when it tried to buy US Air. Regulators, unions, and stock holders all fought it viciously.
Code sharing and marketing agreements is the way to go. This allows the airlines to focus more on operations and less on politics.
The part of your premise I do like is that both carriers stand to gain from more long haul international flying. The new Boeing and Airbus planes will open a lot of possibilities.