MARY SUE CHARACTERS
Also Known As: Marty-Stu, Gary-Stu, etc.
What is it? If you don't know, just go here. It's easier for both of us this way.
Why do Comic Companies Do it?
No, sorry, that's not a worthy answer. Let me try that again.
Really Lazy Writing by bad fanfic authors who pass themselves off as professionals, but have no idea what to do with a character, so they just write a given favorite character as their own mouth piece.
Recent Offenders include but are not limited to:
Geoff Johns (Hal Jordan), Ed Brubaker (Bucky Barnes/Winter Soldier/Nu Captain America), Mark Millar (Too many to count, and it requires understanding Millar Logic, so why bother?), Frank Miller (Batman), Brian Michael Bendis (The Hood, Luke Cage, Nick Fury, Jessica Jones, The Sentry...it's a long list here too), Garth Ennis (Any given Ennis Protagonist), Warren Ellis (any Given Ellis Protagonist) and many, many more.
While this trope has always been common to comics, I'm going to have to (again) point to the Dark Knight Returns as being the point where it went from "occassional but acceptable" to "down right obnoxious and omni-present"
I'll just assume you skipped over that whole "Bad writing" thing I put up above there, just to humor you a bit. Even if you were to ignore that, you are left with the fact that in any artistic medium, sooner or later you have to show and not just tell, and this is true of storytelling. We, the audience, need some compelling reason for the audience to give a crap about a character. If all you do is have your characters talk about how "special" your personal favorite is, and you have them constantly upstaging other characters with no consistent or logical reason for doing so, it's obvious and more than a little insulting to the audience. And that's how we feel about your characters: insulted. Not a great way to keep us coming back, is it?
Angles for the Redemption of the Device?
See that poster of the Sentry up there? yeah, that works for me just fine. How about you?