Pre-publication, the process consumes lots of time and energy and money while generating zero revenue, output, or other benefit. Very different from most business ventures.
Source: I have written a book, and have also started my own business and have worked for many startups. (VC-backed startups pay their employees.)
Being a paid employee of a start-up is different. That would be similar to receiving an advance from a publisher to write a book.
Not all investments are created equal. the bigger Startups convince rich people to give them millions to work on their idea. Publishers completely invert this; you show them a product and you get a piece of the pie in exchange for them distributing. One's a much safer bet even if everything falls apart. And in the corner, entertainment is pretty much the biggest gamble. Always was, probably always will be.