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?
> Is there a rationale for leaving money on the table?
Sometimes I just want to do a project for fun and plop it on Github and see people's reaction. Not everything has to be monetized, and that can sometimes become an ulterior motive: 'If I keep releasing stuff with monetization I will profit' mentality creeps in.
What keeps aeroplanes in the air? Money.
And what keeps the money coming in? Thousands of man-hours of sales, legal, tax, operations, and finance. Like someone earlier said, code is the easy part.
Lots of people have side projects as hobbies. If you start charging for it, the hobby becomes a job -- which pretty much eliminates its value as a hobby.
I already have a job, why would I want a second one?
Coding is easy, selling is hard.
Money and thus contracts add pressure. Suddenly a hobby for some evenings becomes a 24/7 operation where you need to have your laptop ready during holiday.
Expectations rise. A fun website now needs terms-of-service, privacy policy and all kind of other legal setup. Sure, it's mostly boilerplate but an unhappy customer is more likely to sue for damanges if they believe you're a company (and thus have money) than a free no-gurantees side-project.
Tax setup should be easy but it can be an annoyance especially if you already have a full-time job. Some people don't like dealing with taxes at all (I used to be one of them).