Because in ancient times, filial piety was very important, and growing a beard would make you look older. In the past, people with long beards were generally old and prestigious. They are respectable people.
Young people look old with beards, and secondly, they feel unkempt and a little sloppy. At the same time, there is another very important reason. My father grew a beard when he was alive, which was a disrespect to his elders.