Consider a person standing still on the ground and a car, which is moving with a constant speed $v$ in a straight line. From the frame of reference of the car the person is moving backwards with same speed $v$.
What causes to the person to move backwards in the frame of reference of the car?
In the case of an accelerating frame of reference we can assign a pseudo force but in this case the acceleration of our frame of reference (i.e. the car) is zero, so we can't say that pseudo force is acting on the person on the ground.