Because VehicleSeats should be placed at the center of the car for best effect.
They work out the distance to the wheels involved and calculate the speed that the wheels should turn at but if it isn’t centered that happens.
HingeConstraints have been deprecated but they still work since older cars still have that system.
Have you looked at the Car kit tutorial?
It allows you to move the VehicleSeat to wherever you want and also provides realistic steering and suspension using Constraints. Remember to follow the steps very carefully and watch the videos to see what you need to do.
You may also want to check the Friction Property of the wheel Parts. I usually set it to maximum (2) for wheels to provide some traction when going up hills or around corners.