It's because GT5 is half year old and everything has been made there. Shift 2 is new and has new challenges and tracks. That's the only things keep me playing Shift 2.
As for physics in Shift 2 it's nearly non-existent.
It's a way worse than in GT5. Bots have very high weight (comparing to player's car) and sticky tires. No matter if you touch bot or bot touches you, you will spin off the track while bot won't even change direction and continue to move its way. Bot easily can overtake player by driving on grass. Bot are unaware of player completely, if they need to move to side, they will do this regardless if player occupied this place. Bots are like trains on railroad. Driving in Shift 2 is just driving on worst line just to avoid bots. The only strategy in Shift 2 is to get first position at first moment of competition and then you will be able to follow best line and eventually win.
Of course, after this disastrous driving in Shift 2, you will see how GT5 is good to drive and enjoy it