You're watching a competition.
Why would you go to watch a baseball game, or a football game, or pay to go see a concert? You are going to watch two teams at the height of their field compete against each other. You're normally doing this with friends who are also into gaming, its a social experience where you get to watch people way better then you do something that you enjoy.
Personally I don't go to them live, But i was in discord with a group of friends and we watched the finals just like we would watch the superbowl. We understand the games so we watch these pros and do the same exact stuff we do with physical sports, we second guess the players, the playcalls, the choices and picks. we oooh and ahhh at good plays, and we laugh at bad ones.
I agree that there will be a group of people that watch it. I myself play a lot of video games. But to me, watching a videogame match is the same as watching a chess match. A lot of people do it, myself on occasion, but it is just not a good spectator sport.