@waezi2: I'm guessing it's because they want heroes who have very little history, so that they can shape them perfectly to fit into the movies, similar to how they did with the Guardians. Plus most of these new characters are either women, minorities, or have a very unique trait (like Starbrand) and they want more characters that can appeal to more demographics.
This makes a lot of sense. Marvel (and DC) keep trying to add diversity to their rosters and this is Marvel's solution to a rather large problem that comes up when they attempt to add new minority characters in their world. That problem being that for seemingly no reason everyone with powers who initially appeared around the world (FF, Avengers, The X-Men, Spider-Man, etc) were all straight white guys and even in universe there was almost no one with powers except for straight white guys (with exceptions of course). Then all of a sudden there's an insane burst period about 10-13 years into the universe's history (in-universe, not real-time) where all these minority heroes started popping up out of nowhere.
There's no real way to explain that discrepancy, so Marvel can solve that problem by retroactively inserting the characters: "There were always minorities helping to save the world, readers just didn't know".
I'm okay with it, it's the best solution to the problem and it doesn't do any harm to the universe (unless the writing is terrible, but most introductions of this kind I've seen have been done rather well), if anything I think it just makes things more believable to have minorities running around.
OT: Yeah, they do it a lot, but I wouldn't say it happens too much.