Cnet's got a story about how the Sims franchise has shipped 100 million units. If you assume sales at around $30 per box as an average (to account for folks purchasing from the bargain bin), that's... $3 billion.
For a franchise whose core concept is basically playing dolls.
Something else to keep in mind here is that for a "triple A" title, selling 500,000 units is considered Pretty Darned Good(tm).
Maybe there's more money to be made in letting players dress up their avatars and "socialize" with others than there is in machine guns, misanthropic aliens, and grumpy dragons. Or maybe the Sims just did an excellent job of appealing to a wide range of casual players by not really having any objectives in the first place. What will be interesting to see is if this is repeated with Spore (which should be released later on this year).