Why do hiring managers rarely offer explanations to rejected candidates?

Will you commit to telling everyone, from here on out, the honest reason why you're rejecting their invitation? (Everyone! No exceptions!)

Will you tell that one guy that you won't grab dinner with him because, to be honest, you found him a bit creepy and plus, he's unattractive? Will you tell your Facebook friend (who keeps inviting you to stuff, wtf) that, uh, you barely know her and the one time you met, you found her kind of annoying? Will you tell that friend of a friend you met that you aren't going to go grab a drink with him because he's just not that interesting?

Nah, you'll make up an excuse. "Oh, so sorry! Would love to but I'm busy that night. Next time!" Or maybe you just don't write back at all. So much for honest feedback.