The physics going into a car model are more complicated than an individual player in say EAFC. The "cost" of something going wrong is also more significant. If you hand over the car to the AI in a situation it's not prepared for and it causes a multi-car pile up and brings out a SC/ RF that has significant implications for the race result. Compared to in say EAFC when you change a player, most of the time the player you're leaving behind is no longer close to the action so anything silly they might do while the AI sorts itself out has minimal impact.