Karting in GT5 is more of a gimmick at the moment than a proper modelled physics engine. Karts are quite unique regarding how they behave and it's a black art just to build a good one, let alone simulating it. You have chassis flexing, seat flexing, solid rear axle flexing, ever evolving engine dynamics, and an actual human body moving about. The way you sit can change how the thing behaves, in that sense it's more like a bike than a car. So for the guys at GT5 to get it anywhere near right they'd have to invest a considerable amount of resources that they just wouldn't see a return on. It's a bit of fun in GT5, and it's nice to see in there.