A joke exists in the aerospace world of "what keeps aeroplanes in the air? money" which feels generally analogous to software projects' long-term survival & prosperity.
Is there a rationale for leaving money on the table?
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).
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?
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.
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.