In addition to what others have said (you need to understand the game, not be an actual player of it), many channels cultivate a community of the same regular viewers. While gaming is technically the thing that brings them all together, the streamers' humor, community in-jokes, and a sense of shared interest are what make different channels unique. As a non-gamer, that's why I watch a few of them regularly.

Watching streams is "about" gaming in the same sense that tailgating is "about" football.