Because of economic downturn in the world, most people have become jack of all trades and master of non. learning skills and not getting a mastery of them because they are testing waters on which of them will eventually pay the bills. I have a friend who during her spare time learnt soap making, baking, sewing yet she's not making money off these skills.
Learning a new skill doesn't always mean you need to make money out of it. Maybe your friend has enough source of income and those skills you said were just his hobby.
I have a hobby of cooking and recently been learning different type of dishes to cook. My friends and family said I am a good cook but it haven't crossed my mind yet to make it as my side hustle.