I know they don't.
The main logic behind this is that Visceral Games (the developer of The Sims 3, formally EA Redwood Shores) simply doesn't have the resources necessary to develop The Sims 3 base game and every single expansion to it AT THE SAME TIME. If they had, then they wouldn't have been expansion anyways. They would have just been the base game.
The Sims is not the only game that Visceral Games works on. They're also developing Dead Space: Extraction for the Wii and Dante's Inferno.
It sounds to me like sims3isajoke doesn't understand how game development logistics works. Making content requires time, money, and manpower. Unless Visceral Games can make all three come out of nowhere, what sims3isajoke is suggesting simply isn't possible.
(EDIT)
And yes, I know that The Sims 3 isn't on
Visceral Games' website , but if you look at the credits in TS3 it quite clearly says EARS, which is EA Redwood Shores and EARS was renamed Visceral Games shortly after TS3 went gold.
The reason TS3 isn't on their website it because it doesn't fit the theme of the games they're currently working on.