The 'because people keep paying (and playing)' answer has already been given. It's one of the most important reason and part of the argument, methinks.
I gave up on it with two prepaid months on it left, but I felt it to be important to strip naked and hand my metric ton of gold and few items of interest that weren't magically glued to my character to someone still doing in-game time.
WoW was a fun experience when it was fresh, but it quickly degraded into both watching a social experiment unfold and witnessing the addictive force of digital crack cocaine in action. Its addictive qualities are easily another important reason for it to still be around, and it makes me wonder what is to come after those, uhm, Pandas.
Really, don't get me wrong. I enjoyed my months of WoW, but I'm pretty positive I would never again want to pay cash to be given a subscription that allows me to play online multiplayer. It just doesn't feel right after years of WoW drama and asshattery.
Come to think of it, crApple's App Store works off pretty much the very same basic principle of customer retention. Stop paying for access to WoW, and you'll lose months (or meanwhile years) of hard work getting all those player characters of yours levelled and geared up. Switch to non-crApple products, lose all those Apps, pay for them again at some other providers digital App-peddling outlet.
You pay to keep your fidelity going. Up until the 3GS, it made sense to me. The very moment they started peddling a mobile device that has glass on the top and the bottom, has a shorter battery life and worse reception on both wireless and phone networks, I started to seriously doubt the ingenuity and sincerity of Apple.
You don't get that with WoW. I no longer recommend it, due to its evidently addictive nature, but I get why so many people/kids want to check it out. In a way, it could still be used to train teenagers and introduce them to the repetitive nature of most forms of employment and, at best, help them to get organized (plan raids, work on common and personal goals, stick to the plan, etc.). At worst, it's still potent enough to turn even the brightest kids into addicted, antisocial shut-ins.
Yeah, I'm still fascinated by it. But I'm more of WarCraft/StarCraft kinda person.