that's like asking why is the sky blue
the sky is blue because god loves the infantry.

I enjoy PvP servers for just that reason. Where is the challenge in killing AI? I like killing the AI while watching for PvP opportunities.
its all personall preferenece. if i decide i want to beat up hordies, then ill flag myself or queue in for a BG. if i dont want to do pvp, then i shouldnt have to bend to the will of someone else who does. i had my fun with pvp, i did it all the time before they changed to the new system (i then got pissed cause all the work i was doin had been lost imo), and it was fulfillng. but since i never played much during BC (until recently), the time i spent doin heroics and raids were more than enough of a challenge for me to be happy and feel accomplished with myself.

Risk vs. Reward
and raiding has better rewards than pvp anyways.