The reality is, it's self validation for most of the viewers. So when they see their message or subscription hit the screen, they can say that it happened and fell good about themselves.
The people that actually get their comments read on a big stream is even rarer so they take it as a "badge of honor".
Trust me, I don't get it either but I know how people act when it comes to stuff like that. I just watch it most of the time and will comment once in a blue moon (normally to another comment and not the streamer).