Why Israel isn't invading Gaza

With Kassam rockets being shot at Israel's western Negev region every day, I know a lot of you must be wondering why Israel has not launched a massive ground operation into Gaza to clean the place out. Until today, I thought that it was because Olmert just didn't have the you-know-whats to do it. But it seems that there may be a more sinister reason and like much else that goes on in this country