In terms of turning passions to profit, it is way harder to earn from a hobby that was self taught than to earn from other skills or jobs or as an employee under a boss.
I think I believe on the contrary that earning through your passion is harder than going to work under a boss. For me passion is easier to earn because you enjoy what you do and you can do more of it as it is fun for you and if you do more, it means more money for you. The point is just very few people have realized the passion they have and lack the ways that they can generate money from it.
If we want to understand that it is easier to make money from passion, just imagine where you have the hobby to write a letter and someone is offering to pay you to write a letter for them. Or where you have the flair to make people laugh and you choose comedy as a career, you will see that you will easily excel in the comedy business than those who went to learn through reading out script. People enjoy those comedians who are naturally funny and they can pay any amount to attend their shows.