It's probably for the same reason there's no replays in 2 player split screen mode. It's one thing to have an bunch of premium AI cars on the track, it's a completely different thing to have two human controlled cars on the track on the same screen. When you're online, each PS3 is running one car. The Cell Processor is a Networking Chip that means that when multiple machines are connected via internet, they pool their resources, and thus a 16 player premium car race is possible. However, it's probably VERY hard for the PS3 to handle the graphical detail of 2 premium cars controlled by human players on the same screen at the same time. This is probably also why there's no cockpit view in split screen: too much detail. Essentially there is more detail in the cockpit of the premium cars there there is on the outside. However, it apparently consumes so much power just to render the exteriors of the premium cars that there isn't much for anything else in split-screen mode. I wouldn't even be surprised if ALL cars in split screen mode are standard models, just to give the system a break.
