Perhaps the question itself is evidence that the game is not going to appeal to me, but I still want to understand.
Most games have a linear progression with a satisfying conclusion. Or at least, that is the goal. You play, you learn, you have fun, you get a reward. Even games like World of Warcraft and Diablo 3 have a storyline that reaches a conclusion, and encourage continued play through rewards such as stronger gear. What sort of reward does one get out of playing Overwatch? Is it just about winning/losing matches? Do you get better gear? Is there a story that I'm missing somewhere in all the media available?