I think that it's because he's human. I'm not saying that he's not TREMENDOUSLY flawed, but that just makes him more endearing and relatable.
Also, he used his INTELLECT to create a machine that enables him to defend and protect people. He doesn't turn into The Hulk, or have superhuman powers. He's just a man.
But I like you too, Steve! Er, Mr. Rogers! More people could stand to be nice and muscular decent like you.