What puzzles me is why, with their $237e6 budget, they couldn't afford a decent script.
To some extent, it's always been this way. Look at Shakespeare: most of his plays are set in foreign countries which would have been terribly exotic to his audience, or are concerned with battles, magic, or murder - it's all pretty sensationalist stuff.
With this sort of big-budget production story complexity takes a back seat to putting the audience 'into' the experience. Gattaca is a fine sci-fi movie that turns on a clever script and strong acting performances, but it wasn't exactly a box-office sensation. But it was pretty cheap to make. with something like Avatar you want a story that everyone can follow easily rather than one that risks alienating any segment of the audience. Epic stories generally try to minimize ambiguity and complexity: the payoff is emotional rather than intellectual.