Do you think, for one minute, that Bird and Trane didn’t "cultivate" their individuality? Every player sets out to find his/her voice and does so through a combination of inspiration and the hard work of trying new things; everything from developing harmonic concepts in their playing to trying fifty different saxophone mouthpieces to get the tone that they hear for themselves in their head. Bottom line:
Although the above is true, it contains in it, the inherent belief that any musician who did these things could be as good as "Trane" Miles or "Bird"; that's not true. Regardless what you do, if you were not given at birth, whatever differentiates those musicians from the rest of us, you can only be a good sideman, or mediocre leader.
"You can be what you want to be"; that's true only to a certain extent; we are talking about famous jazz musicians who were the best; you can only be the best if you were given something at birth, that for lack of a better word is called "talent".
Within them, the very best jazz musicians know that they have something that's so exclusive, that very few will be able to duplicate what they can do, and are doing; they know this even if they are at the bottom financially. While that doesn't suffice for "In God We Trust" in the pockets, it's what keeps those musicians who are not financially successful, going on another day. Read what Miles had to say about "Bird" to verify that.
In regard to "individuality"; that is something musicians "discover" while in the process of becoming a good musician.
Enjoy the music.