There may be two reasons: first, from the habit of speaking, I usually like to speak loudly; The second is to pursue immediate results. It is true that yelling is more effective than whispering at some point, but this method is not good for children's future growth.
Generally speaking, children's behavior does not conform to their parents' wishes, and it is easy to arouse their parents' anger. They are required to stop this behavior immediately and obey their own orders, so the volume will be raised, hoping to have a deterrent effect on children. Children may choose to obey because of temporary fear, but it will not last long and will show their true colors.