By playing against other people, they're playing another game that uses DOOM graphics. Yes I've played it over the network, but it wasn't a lot of fun. It's like baseball, it's fun to swing at the ball, it's mildly amusing to catch balls launched into the stratosphere or flying at you at high speeds, or even to see how fast you can throw the ball straight.
Playing a game of baseball is incredibly boring, you get to bat maybe three times, more often than not told to not try to swing for the fence but to just get on base, even if it means jumping in front of the ball to get a free base (taking a ball to the shoulder at 90mph is not MY idea of fun, ymmv), because statistically home runs are unlikely, but a team that gets players on base will almost always get the most runs. Only one guy can be the pitcher so the rest have to stand around the field being bored, waiting for the ball to get near them. Mostly it saps all the fun out of the game.
The same situation exists in football, soccer, and basketball. In my opinion eSports are even worse because the single player game was designed to be fun, while the multiplayer game mechanics tend to deviate very far and you find yourself doing annoying things just to win.