Primarily because whether you get promoted or hired for a position is in someone else’s hands. That person might be discriminatory, they might want their friend to have the job, they may not like you personally etc. so in spite of how hard you may work and that you can do your job exceptionally better than all other candidates, it can be irrelevant if the person promoting/hiring doesn’t want you to have the job for their own personal reasons? Not sure how common this situation actually is though