Because I believe in promoting integrity in martial arts. Again, I think it is unseemly to proclaim yourself a 9th or 10th Dan in a style you founded or an organization you founded. Doing so implies you have a big ego because a true Master would never proclaim himself 9th Dan. You are 9th Dan, or whatever Dan, when your peers or other Masters, who do not have a vested interest in seeing you promoted, say you are whatever Dan. I will be 5th Dan when A. my Instructor decides I have met the criteria to be recommended and B. when other Masters who do not care if I pass or fail judge and recommend me (or not) to pass 5th Dan. I will not be 5th Dan when I decide I want to be 5th Dan.
Let's say, hypothetically, I decide to found my own style. Even if I develop all the forms, develop the curriculum, decide how it will be taught, and begin teaching, I am still not a 9th Dan in that style. I am a Dan holder in Tae Kwon Do or X martial art who founded a style based on ideas I wanted to pursue. I might refer to myself as Head Instructor of that style until I retire and pass the duties onto a senior student or students, but I am not 9th Dan. Why? Because it is unseemly and egocentric to declare yourself a 9th Dan in an art you created. Does it happen? All the time. Is it right? Nope.
For the record Won Kuk Lee, the Father of Korean Tae Kwon Do, never declared himself 9th Dan in anything. He taught based on what he had learned over the years, but never promoted himself.