there are flight simulators, train simulators, and maybe other things... i'd say flight sim's are the most logical, as flying seems really fun, (and of course multiplayer basicly degrades to dive bombing each other, or... atleast for my 9th grade aerospace class). on the other hand, train simulators... compared to driving or flying, i dont see there being much to do. but like flying, it's something that people dont do every day, so that's the angle i can see them comming from to make that game.
while car sims seem like there would be more to do then train sims, it's something that most people over 16 do anyway, so it's got that much less of a reason to make it.
i dont know how many people want to drive home from work to get on a computer and drive some more... if the game was a real driving sim, it would punnish you for speeding, hitting stuff, etc. while running from the police and other stuff would be fun in such a realistic game, there's other games out there that reward you with a point system for that... AND, having tons of good looking car models would really really tax most computers, especially in stop-and-go traffic conditions. and a car sim without traffic would be pointless.
basicly, it may happen eventually, but... it's really not one of the game ideas that anyone want's to jump on making, when they can make something like GTA instead.