It's because everybody knows a WoW player who IS a lifeless nerd.
Not that all lifeless nerds are WoW players or that all WoW players are lifeless nerds.
Stereotypes are not facts, but just an observed trend, whether or not the trend can be generalized. Trends are reinforced whenever someone sees an example, and there are plenty of them. ESPECIALLY when the example is extreme.
When I think I WoW I think of 2 friends. The super smart guy who aced highschool, got a full scholarship to my college...and flunked out to play WoW, and my smart roommate who tanked his grades in college from WoW and landed a pitiful job. I also know dozens of other people who've played WoW and nothing happened. But what comes to mind are the 2 guys where something major /did/ happen.
Heck, I've played WoW and found it to be a great game! It deserves the popularity it has. Personally I hated it and quit in about a month. I seriously damaged my grades as well, but with City of Heroes, not WoW. But WoW is the standard-bearer for gaming's excesses because it's far more prominent. It's just part of the burden of success.