That applies to occupations, not to hobbies. I.e. those who can't find a proper job using their skill go on to teach that skill.
It does not however apply to the most valuable content - one made not for money, but out of love for the subject. A lot of masters in all occupations simply like to share. Our industry is probably the best example - it's almost entirely built upon masters who gave away their knowledge. But it happens in other industries too.