Why is the button on the collar of a T-shirt called "doorman"?

This is called "door guard", also called entrance guard and front guard, not "doorman". The following is a detailed explanation:

Foreclothing generally refers to the part where clothes are buckled or zipped at the midline of the human body. The front placket of a coat is also called the front placket, that is, the opening of the front chest from the head to the bottom. The placket is the most striking main part of clothing decoration, which sets off a fashionable and gorgeous appearance with the collar and pocket. The styles of the front are endless. The front of trousers usually refers to the front of men's trousers or jeans, that is, at the front of trousers, a fork is opened from the waist to the front crotch, and then a zipper or button is installed. Men's trousers are designed with a placket to facilitate men to urinate, while traditional jeans have a placket, so both men's and women's jeans have a placket.