The higher the pressure, the more the sealing elements are pressed together – and the higher are the forces on the ball and the seat.
This would not be a big problem, if the valve has not to be operated like a check valve. But a ball valve has to be operable, even under high differential pressure.
When high forces are acting on the sealing elements, the friction between them is high. This means that the operation torque and the wear on the parts are also high.