Because you write does not mean you are special. For every alive human being, there is not one who does not have original and poetic thoughts. Perhaps not all the time, but to those few souls who do spend time and effort to twist and blend their their ideas and opinions together to make astonishing reading, it is still a formidable avalanche to complete against others.
Which is why it is absolutely necessary for you to give yourself the best chance you can by obeying 'the mostly unspoken rules'. Which are thankfully simple. If, as a creative writer, you believe that you are beyond the mundane and the quotidian, that you believe, if on only the merest level, that you wish to work at a more thoughtful pace, with less artistic compromises and that you will not bow to the 'money changers at the temple', preferring to dedicate yourself to idealism, then, unless you are a remarkable genius, you will fail.
Every item, word, sentence, paragraph, letter and script which leaves your hands must be, not as perfect as you can make it, but perfect. Period. For that which you send out says everything about you. On some level, the recipient will have the ability to 'read between the lines' and if you have not given the piece enough thought or care, they might come to believe that you are lazy, self-indulgent, scatter-brained or any other number of ugly conclusions. Certainly they might arrive at the conclusion that you might not be good (fun) to work with. Remember, if nothing else; everything you write says something about you.
Learn the proper rules of the language in which you are writing, become a skilled grammarian, use a Thesaurus and never believe you are good enough to not need one. Develop respect and humility for all you write for. Learn manners. It’s a simple enough premise. Treat people like you yourself would wish to be treated. Like you, although they are not special, they believe they are.