The only one I've ever heard is "security." Sometimes people try to "yeah it's because for security we don't want some tool to paste credentials or clickjacking mumble something malware hacking clipboard spyware javascript browser" but most of the time the truth is "our sec people need to show controls for hardening the authentication process so they came up with that and we just did it because it wasn't worth the fight"
I've also heard "being able to copy/paste defeats the purpose of having a 'confirm email' field." I reject that but it's at least somewhat logical.