Question

Why can't we use a concave mirror as a rear view mirror?

Answer 1

This is because the concave mirror will magnify the object and we will see a very enlarged image.
but in convex mirror it converges the rays and shows the correct image
In case the driver wants to turn or stop a vehicle he should be well aware of position of vehicle following him and give a signal accordingly so as to avoid a collision. The convex mirror reflects better view of pictures behind him due to a vide viewing angle of reflection.