You can never rely completely on your rearview mirrors when changing lanes because

The reason why you can't rely on rearview mirrors when it comes to changing lanes is because using rearview mirrors has also blind spots where in there are area you couldn't see and that's where side view mirrors comes in as they are vital in terms of changing lanes for it could provide the view in the sides of your vehicle when the rearview mirror lacks as it is mostly focused on the vehicles on your back.