The reasons are:
1) People have different tastes. For some people, modding a car to go 300 mph is their idea of fun, but it takes the fun out of the game for me.
2) GTA has infinite gameplay possibilities. It would be impossible to cater for them all, and even if you could, MTA Blue wouldn't come out until about 2010.
3) Also, Blue's not going to be some half-hearted shell where you have to program your own game mode. They said that there will be many game modes shipping with Blue.