Because ancient people believed in the concept of "body gives parents skin", ordinary people were not allowed to change the length and style of hair or beard privately, so hairdressing belongs to a special industry, so it has a high social status.
It is also because of the reasons related to beheading in ancient times. As we all know, beheading needs a knife, and so does haircut. Therefore, cutting someone's head is a sign of disaster. In order to avoid the influence of this taboo, therefore, the barber industry is not allowed to shout loudly, but to knock with two special iron plates, which are thin and narrow, and the sound is crisp and long, and can be heard from a distance. You know it's a haircut as soon as you hear it.