For all the same reasons as people watch sports commentator shows?
Game videos only tend to have the player in a corner anyway, it's not like you're only watching their reactions. So it's more like watching sports, which are also being commentated.
Except with games there's a lot more personal interest. Unlike vegging out on the sofa watching the footie and shouting occasionally, you may be deciding whether to buy the game and play it yourself, or learning tips and tricks to get better at a game you already play, or you've finished the game and don't want to go back and play for another 40 hours to discover all the secrets and hidden things and alternate endings.