Same reason people watch any streamers; they either enjoy the streamer's personality, the games they play, or both.

For a lot of people, it's basically like watching one of your friends play a game you enjoy, and a way to become part of a community of others interested in the same thing.

Meanwhile in other cases it's a matter of just not having the time/energy/skill to play the games they're covering, but being interested in seeing how they pan out. For example, I often watch people play extremely difficult mods of games like Super Mario World, Super Mario 64 and New Super Mario Bros Wii. Yes, I could easily go ahead and do the same thing, and I do play mods of those games which are a lot easier...

But I find a good player's experience of these games fun to watch, and don't feel like spending hours on every level trying to match that myself.