They get more viewers when they do. Streamers do exist who don’t use a webcam, don’t talk to viewers at all, and only show gameplay. They get some viewers but not nearly as many as they would otherwise.

People like interacting with streamers. They especially like seeing the streamer get excited when they donate a bunch of subscriptions or send a bunch of money. Popular streamers will even have animations and sound to make a big deal out of this.

The truth is that full time streamers are supported by a small minority of viewers who donate large amounts of money. This makes streaming somewhat akin to prostitution in my view. But there you have it!