I find two issues with this (disclaimer, My "hobby" is coding, so I guess I've been monetizing it, all my adult life):

1) Once someone starts paying you, they get to exert their influence on your work, and it becomes a lot less "hobbyish."

One of the biggest joys in my life, these days, is not having managers destroying my work.

2) Delivering ship software requires a lot of "not fun" stuff.

In some cases, the majority of the project could be wrapped up in "unfun," like needing to use particular coding practices, languages, dependencies, techniques, etc. Also, there's all the "polishing the fenders" kind of thing, like documentation, UX finish, maintenance, fixing all those bugs that you don't think are "actually bugs," establishing CI/D, configuration management, presentations to the Board, etc.

These days, I write software for myself. I eat my own dog food. Every one of my projects is a complete ship product, with all the aforementioned stuff (without the CI/D, as I don't really need it, for my one-man shop).