Shift, forza, many other games have no problem having Ai thats a challenge................... Play more games.
Sales would plummet if they made Gt online online. Millions buy the game for Sp alone....
Gt4 had 500 events, Gt5 has the smallest , worst career mode out of any major race game of the last few years......
SP will always be a major bread and butter of any major racing game and for good reason. GT5 fails in this area and needs to add more events, championships, manufacturer events, different laps, etc... most of the cars are not even used in game.