The Avengers only lasted a full three issues before Captain America showed up. For all intents and purposes, he is a founding member. In fact, he was even given 'founding member status' in the books.
Don't ask me how I know that.
Anyways, he's probably more closely identified with the team than even Thor and Iron Man are. And you can't have a multi-million dollar blockbuster Avengers movie without Cap. Not because of a mere three issues.