Zodiac (2007)

How exactly did David Fincher get a major studio to produce this film?
It has no action, no dramatic arc in the usual sense, no oscar-baiting melodrama, but oozes a completely unusual dry weirdness. The whole movie stays absolutely clear, open, matter-of-fact, but feels cryptic, puzzling and slightly disorienting and, at the moment, I haven't got a clue why this is so.
So, for the time being, I'll have to call it an unexplainably great film and shut up.