Why should I teach others to do something instead of myself, when I can do it faster? It would only be a waste of time. Especially new managers, but also stubborn old bards who are afraid of losing their authority, think this way.
A manager who does not delegate only makes his team feel bored and they stop trusting him. He becomes exhausted and may even burn out. As a result, he will not get more, but less work done.
Apart from getting work done, managers should also be interested in developing the skills of their subordinates, that is why they should be able to delegate.
Delegating does not mean only assigning tasks. It's a tool to ensure that each member of the team, including the manager himself, can contribute to the company's business goals and constantly learn new knowledge and skills.