Sometimes watching a terrible gamer struggle, and listening to their exasperation, is more entertaining than watching some ace just zip through the challenges.

It's a similar community thing to that "watching a friend play at their house". So many of the biggest streamers may or may not be "good" at the games they play, but certainly work their tails off trying to seem the most like "a friend", their channels the most like a friendly community or even "their house". You want to see them succeed because they are a friend, you enjoy watching them suffer sometimes for the same reason. The social nature of Twitch chat shouldn't be easily dismissed. It's easy to laugh at the "rituals" that have developed where almost every streamer is constantly interrupting themselves to "thank" chat users for random things that matter on the platforms but not really outside of that (follows, likes, "sparks", what have you), but those are absolutely social interactions evolved (tough to say designed in some of these cases) to try to make "friends" visit "your house".