I'm not a twitch streamer/viewer, but I believe a lot of folks are in it for the community aspect. Following your favourite streamers and being a participant in the group chat - especially if your streamer chats with the group - is a way for folks to feel like they're hanging with friends.

In contrast, watching TV by yourself can be quite lonely.

There are a lot of other differences between TV and twitch; I'm not sure how much worth there is in comparing them. They seem (to me) to mostly be different categories of products that happen to both include a video component.