Because interacting with streamers is a big part of Twitch?
I love popping into a nearly-empty Twitch stream and interacting with a streamer. It must be pretty discouraging to stream into the void with no viewers, and it's cool to hear about why they like a game I'm considering, or getting help with learning a game, or (rarely for me, lol) sharing some of my knowledge about a game.
It's also the only way I connect with two of my (married) friends from uni. I pop into their stream when I can and we chat a bit while they play.
Twitch with no chat might as well just be a YouTube video that I can speed up, pause, and seek.