Why can't Marvel be genuis and cast AMH as Captain America?

Ever since I read in an old magazine 5 years back that Robert Downey Jr. was going to play Iron Man. I thought "Perfect! Now, all they need to do is get AMH to play cap!

With all these new Marvel movies supposed to be building up to an Avengers flick, it makes perfect sense to me.

Why?
+He looks the part!
+Iron Man and Captain America are the leaders of the Avengers. They are peers and banter with one-another would come second nature to these two actors. (AMH and Downey)
They are around the same age.
They are actually good friends, and could play the banter part together without any effort.
Having an actor that is too young for the role will cause problems with the best parts of the character.
CAP has always been OLD!
+They are gonna CG most stunts anyway, so might as well use a guy that fits the looks.


Problems with it (not too many):
In the years since the brat pack, Robert Downey has grown as an actor and has shown that he has got plenty of talent, where AMH has not so much.
Although that could just be that Robert Downey has gotten better roles.

Anyway, this is what I wish. And I know I have not done much research on the subject, but I saw the Iron Man movies, and used to read Avengers and Captain America as a kid. And I just want to see if anybody else has an idea about it. Want to debate?