Natural musical abilities sustained by a huge amount of work
A certain number of well-known musicians in popular music genres were never able to read and write music, however that did not prevent them from producing the results that we know. It has nothing to do with failure: they simply did not need to learn how to decrypt and notate music to create it. Yet they are highly appreciated music composers or performers.
How do they do it? First, composers and performers without formal training are extraordinary music listeners. These musicians keep their ears open while they are playing, alone or with other musicians, and remember all the musical elements they find, practicing and listening all the time. For some decades, technology has helped. For example, Michael Jackson did not read music, but apparently it helped him to extend his naturally generous vocal style, and he simply used a tape rrecorder to remember the melodies that he was singing a cappella, before recording them in the studio.
Why do some musicians want to get a formal training in music while others do not?
The main reason why some musicians do not want to study music theory is that they do not want to get distracted by written symbols. It is neither better nor worse: it is different. They want to focus on what is the most important thing for them: producing powerful sounds. Concentrating on aural learning, they conceive and execute musical ideas in a more spontaneous way, and it is their way to find more original and unique ideas, watching, when they are not playing alone, the hands of other musicians in order to communicate with them, etc.
It is just a question of choice: during the 1940s, Miles Davis was enrolled in the Julliard music school. Julliard is well-known at the global level to have trained musicians like Henry Mancini, Barry Manilow, Chick Corea, Wynton Marsalis, John Williams, Renee Fleming, Steve Reich, Eric Whitacre and a lot of other composers and performers. But it did not suit Miles Davis, who chose to leave and followed a less institutionalised path to create music from a blank slate. Knowing music theory does not reduce musical creativity at all. It is not appropriate for everyone, that is all.
Actually, the most important thing is to remember that there are many ways to learn and understand music: human intelligence had different forms, and it is not dominated by a unique general ability, be it in the field of arts or science. Musicians identify for themselves what they learn best, how they learn it and why. And sometimes they like to start venturing into uncharted territories, speaking to other fellow musicians to be understood rather than to be correct.