Well, that's kind of because The Dark Knight doesn't have a green behemoth that can carry objects over 100 tons, a suit of armor that can shoot repulser rays and fly, a god who can fly and wield thunder, an army of aliens from a black hole purring from New York, a giant flying base holding thousands f soldiers and weapons that is held afloat by four massive rotors, advanced space-shipsand weapons of mass destruction beyond our comprehension, the city's destruction, and outer space planets. Explain to me, how could ONE of these things be done without CGI?
Comparing The Avengers to Batman is like comparing an apple to an orange. They're both fruits (In this case, they're super-heroes), but they clearly have vastly different genres, characters, stories, etc. The Avengers is your traditional team-up with super-powers and highly advanced technology and races with the threat on a global scale while The Dark Knight takes a realistic approach with people who don't have powers and are trying to handle an urban threat.
They're both great movies, but you're clearly just abusing the fact that they both feature super-heroes as an excuse that they should have gone in the same direction of development.