Why do we consider mechanical movement and rest to be relative? examples

Whether the body is at rest or in motion depends on the reporting system chosen by the observer. One and the same body in one reporting system can be at rest, and in another it can move uniformly and rectilinearly. For example, a car moves evenly and in a straight line along the road. If you choose a reporting system associated with a motionless person standing on the road, then he moves in a certain direction. But if you choose a reporting system associated with the driver of this car, then the car is at rest in relation to this driver.