We searched and found the most likely explanations... The main reasons for the existence of mirrors are psychological and safety reasons.

When with the industrial development the buildings started to get higher and higher, the use of elevators became necessary. The elevators at that time were quite slow and people who used them often complained. The engineers of the time began to think of a solution to the problem. An engineer opined that elevators are not slow but because people have nothing to occupy themselves with on the way, they get the feeling that time does not pass! Thus the use of mirrors was suggested and the first tests showed that complaints were significantly reduced, so their use was established. People have something to occupy themselves with on the way and thus overcome the psychological scope of idleness!

At the same time, safety reasons require the use of mirrors. The daily use of the elevator has integrated it into our daily life and many times we enter it abstractly. But what if, for some reason, the chamber is not behind the door? There is a risk of falling into the well. Thus the existence of a mirror makes it easier for us to perceive the existence of the chamber as opposed to the dark well.

Also, many people are claustrophobic and being confined to such a small space causes them disturbance. By using mirrors, the space is "doubled" and alleviates the feeling of claustrophobia.