Flies

How come, whenever you've inadvertently left your flies undone, people point this fact out to you? It's not like you're cock's hanging out or anything is it!? If you leave the top button of your shirt undone, no-one says, "Your top button is undone," or if you don't button or zip your coat up, no-one ever points this fact out. So why go on about your flies? Is it a pseudo-Freudian slip, what they actually mean is they're glad your flies are undone, that they'd like to reach in and feel your dick? Is it? And why is the zip or the bottons on the front of trousers called flies? Why not wasps? I just don't understand. Help.