I have a close friend who is a moderator for probably the most prolific League of Legends streamer on Twitch. He spends hours every day watching streams. I thought it was the weirdest thing, as the streamer basically sits and talks to himself for hours while playing games. Then I started watching streams for games I played and I realized how enjoyable it was. Then I got into watching the streams for Competitive LoL games and realized it was really no different than watching Hockey or Baseball (I find the commentators really cringey, but other than that...).
At the end of the day, you're watching a TV/PC monitor for entertainment. Who cares if you're watching I love Lucy, Game of Thrones, Poker, or some dude playing a video game? The more I thought about it I wonder if watching people play games on stream almost taps a nostalgia trip for those of us who grew up watching our older siblings playing video games (my older brother always had first dibs, and eventually I just started to enjoy watching him play the games).