What holds me back is that "monetizing a side project" is another way of saying "running a small business", which is a form of having a job - but I already
have a job, rather a nice one at that. Why would I waste my precious free time working a second, crappier job, when I could do things I enjoy instead?
If I am building something in my free time, it is because I am inherently interested in that thing, because the experience of creating it is enjoyable, and because I want it to exist, for its own sake, because I find it personally meaningful. With that kind of intrinsic motivation, who needs money? Why would I bog a pleasant hobby down with a lot of accounting and paperwork and bureaucratic drudgery when I could focus on the fun part instead, by writing the code I want to write and then just giving it away?