Could be any number of reasons: denial, necessity, fear, uncertainty... but most likely because it takes a massive effort to not take the path of least resistance and finally assert yourself as someone different from the view the world has of you. It takes mental strength and resolve, not to mention that it's something that you can't take back. Once it's out there, it's out there. That is a giant leap whichever way you look at it, and no small feat to accomplish.
There's no shame in wanting to be yourself. Overcoming the vast weight of social conditioning, preconception and expectations in order to realise who 'yourself' is, is something you should be proud of achieving. *hug*