Same question I've been asking. The closest thing I've gotten to an answer is that it's popular.

[Steve smiles at him, but it still doesn't quite reach his eyes. He's afraid to turn his back on him - not afraid of what Bucky will do, but afraid that he'll disappear.]

You used to tell me that coffee would put hair on my chest. [And he shouldn't, he shouldn't. It's not good for either of them.] I guess you don't remember that.