From the publisher's point of view, there's not much advantage to delaying a game. The Sims has a large audience of loyal fans who will probably buy a new EP right away. Delaying it simply means disappointing them and delaying their profits. And they know they'll be releasing patches on a semi-regular basis anyway. Also, delaying the release does not necessarily mean they will find all the bugs before release.
For what it's worth, Blizzard spent around 10 years working on Starcraft 2, and they still release patches for it regularly.
I do think EA's release schedule is a bit unrealistic, and a bit more time spent on the EPs would make them better. But The Sims is a franchise that they're obviously depending on to turn a profit on a regular basis, hence the rigid release schedule for new Sims products. It's a tough business to be in these days, and every publisher needs some games they can count on to always make money. It's not about greed, it's about not going bankrupt (easier than you might think when games cost millions to develop!).
