In this case, the author chose to Self-Publish the book (amazon), and so they'd have to do their own Marketing, Networking, Editing, on-top of writing (the fun, creative part). If they'd had a Literary Agent, it may have worked out - Agent does the heavy-lifting i.e non-writing tasks.

The writing business (books) is extremely competitive - keep a main job, write on the side (hobby) until sales / Agent / Publisher pays enough for you focus on writing full-time.

Stephen King, "On Writing" covers this well, and it's a joy to read.