That couple of untelented programmers at PD with the task to create the Ai forgot about the difficoulty level, which is the only way to fit the game for different kind of players, you're asking why, Imo because they messed up and rushed it. Looks weird after 5 years but knowing PD and how bad they manage their time, it's possible, this is a double face company.
I know everything about arcade mode and I mention it, only I didn't bother to write the whole story so kudos on you, that mode is the only chance to have a decent race, where you are actually racing and not "catching the leader". But if we talk about GT5's career mode we talk about a fail mode, a couple of races could be challenging but overall not good enough, GT5p had a better "career mode" if you ask me. About the seasonal events they could be fun for a while but basically you need to catch the leader from the last place, meaning you are passing other drivers easily, not the same thing of a challenging race starting from 4th and trying to win against fast rivals, everytime I compare GT5's AI with other games, like GTR2, Grid or GT5p I swear so I don't care if I'm unsettling a couple of fanboys in here, PD didn't put enough effort on AI expecially in career mode and the gameplay is suffering.