Because it’s a shallow prog-rock scribble with no faith in its audience. Two and a half hours of one-note characters yelling at one another to explain the blindingly obvious.
Cameron says he started writing this movie starting in the 70s, and I believe him. You can picture him sitting in his dorm room, listening to Tarkus and Olias of Sunhillow until the grooves wore out, doodling weird furry giantess smut, imagining all the claymation he was gonna do to make the bestest sci-fi film since Dark Star.
What the prequels were to George Lucas, Avatar is to James Cameron. This was his stupid childhood dream project. Somehow it made an entire billion dollars. Introspection will not occur.