I like my answer to Fermi best. I haven’t seen it anywhere else; namely, in the multiverse, why should an advanced civilization stay here and fight over this universe with aliens who are potentially much more advanced and dangerous than you are, when you can just slip over into another of an infinite number of unoccupied universes, close the door behind you, and set up shop there?
All you need is the ability to slip over into another universe. I can’t believe it’s taking so long.