Why the Managers do not make/prepare in their team staff leader for the future?

Such Managers are not truly dutiful and competent Managers and are self centered and feel they will lose their position to one if they do so.Managers job responsibility is not only to Manage his team by mentoring and coaching the team,he has to set examples by leading from the front successfully and he has to prepare and mentor and create leaders in his team for tasks best befiiting to each ones capability and if he fails to identfy leaders in his team then he lacks in leadership qualities.