The Japanese arts, over and over, have pointed out a path:
These steps are delineated most clearly by Zeami, who was a great theorist and practical philosopher as well as Noh theater person, but they are demonstrated by other geniuses who have at least reached step 2.
Much of what the great shakuhachi master Watazumido did was pure improvisation, but the emphasis is on the word pure. His technique was so secure -- his pitch so accurate, his dynamics so second-nature, his breath so strong -- that he could allow spontaneity without struggle or thinking about it. Not many people are at that second stage.
Instead many prefer leaping ahead to spontaneity without fully investing in step 1. Of course you can do it this way, becoming an improviser from the git-go, putting together a bag of tricks and working around your limitations. There's nothing wrong with that. As your bag of tricks becomes bigger and your limitations less confining, you may even feel a great deal of satisfaction.
Some of us, however, are enamored of shakuhachi not only for the immediate pleasure of coaxing beautiful sounds out of it, but for the path it provides, an ancient path that may, if we're disciplined, eventually get us to step 2 or even, when we've got some wisdom in us, to step 3. And if you follow this particular path you need a teacher, it's part of the deal.