So do the people who care about stats.
It's the same reason a golfer keeps score. It's less about beating others and more about beating yourself!
Of course, that can't be said about ALL players, but I think for most it [i]is[/i] a fun way to track your progress and actually have a quantifiable stat to go by.
I think you'll find that even the sweatiest, most neck-bearded basement dweller, a guardian caked in dorito dust and with mountain dew exuding from their pores, mom yelling downstairs to get a job, baby crying from the other room being ignored, 7.0 average KD and 99% win ratio, even THAT GUY will tell you he plays for fun.
It's kinda moot at this point.