Why has online gaming become so popular?

On its first day alone gamers clocked up a total of 5.2 million hours of online gameplay on the game "Call of Duty: Modern Warfare", and this is only one game; in April of 2011 there were 77 million registered accounts. My question is this: how has a gaming genre that has no storylines, no learning curve and, in my view anyway, caused game designers to become lazy, become one of the most popular game genres in the world?