Because you want a fast win or a fast loss. You get quite a lot of honour from assaulting towers etc, moreso than turtling at FW and having a game that lasts for 4 hours.

At least that was what it was like before when the ez pvp epixx patch came.

Well you could always ignore the pve bit... let other people do the tower capping and npc killing while you camp the middle with a bunch of friends to take down runners