Since when? That didn't stop Iron Man, Thor, and Captain America from succeeding. It didn't prevent the Bourne movies from succeeding.
Those were origin stories. Start with a team movie and there's a LOT you have to explain to people on powers, abilities, personalities, etc. Or just assume the audience knows this and roll with it.
Would pretty much anything RDJ did in Avengers be even half as entertaining with we hadn't already seen his version of Tony Stark and new his personality? Would casual movie-goers now who Iron Man is, what he can do, his past, why he has this suit, etc.? Who Thor is, what he can do and his relationship with Loki? Who Captain America is and how he's displaced out of time by 70 years?
Without their own origin movies the Avengers either would have had to spend a lot of time setting these people up -which it obviously couldn't do as well as individual movies- or just assume the audience knows who they are, what they can do and just go with it.