But they were all "one sentence" back-stories. Back stories that mean more if you've seen their movies. If we hadn't have had their movies "one sentence back stories" wouldn't have very well established exactly who Thor was. Other than just being a god from legend, we wouldn't know who he is as a person or how he had to grow as one to become the hero he is.
That MEAN more, yes, but don't mean nothing. Not everyone who saw the Avengers saw Thor or Captain America or even Iron Man, or all three of those movies would have been billion dollar films.
So, clearly, people were able to follow the movie without having to have seen the others. Certainly you were rewarded if you have, but it wasn't a requirement.
Do we really NEED to know Thor's journey up to this point? Not really. We need to know that he's struggling with his brother. That he comes from another world. And we see his powers and character in action.
And do you really think Batman and Superman now movies to introduce and explain them to an audience? Wonder Woman doesn't require a two hour introduction to for an audience to get her.
No movie should require a two hour introduction in order to get a character... look at a movie like Oceans 11, eleven characters, explained quickly, easily. Look at The Incredibles... none of those characters had a movie to set up who they were and what they do.