Probably because there is some mildly decent reason (or very good, I don't know) to avoid them and it really doesn't matter enough to worry about getting around it.

Why would you want emojis in your password? It's a piece of text not meant to be seen, emojis are meant to be seen. Just randomly generate some characters and get on with your life. I don't understand why you care about this at all, it's such a pointless thing to complain about.