Because it's a game and games always have either scores, or some other means of determining a "winner".
1) The winner is the one who is left standing after fighting another person / persons.
2) You can only win if the sides were even or you were out-numbered. If you have the numbers advantage, you can have fun and you can get the kill but you don't win.
3) If you kill a building, you can have fun but you don't win.
4) If your side "wins" the war and #2 applies, you reset the map, earned a few perks (hint - you can earn more in the same amount of time by fighting!) but you didn't win.
5) The only person who knows you won is you and the person you fought. Maybe a few around you. Do that enough times and you will earn some respect.
Regards,
Hammer