Having a software product as income source means you don't have to have a day job working for somebody else. It means you can spend your time doing whatever you want, be that working on your own personal projects, surfing, traveling or, yes, earning more money. You can even choose to build open source Mac text editors and give them away for free.
It's your choice what to do with that freedom. The important point is that we, as software developers, are essentially handed that freedom by nature of our skill set.
That's a really cool thing. So while I can see an entitled end-user saying something like the grandparent's "you already have a job that pays you, so give us the output from your nights and weekends for free", it's surprising to see that attitude from somebody who would be most impacted, were that to become a reasonable expectation.
Some people crave the recognition a successful open source project gives.
Some want to learn something.
Some want to give a gift to the world.
Some don't want the hassle of supporting software.
Some believe "from each according to their ability, to each according to their need"
One explanation, like you point out, is that they are free to work on a project such as CotEditor because they are financially secure.
The single greatest benefit of having our skill set is that we are capable, in a way that few other people in the world are capable, of single handedly building products that we can sell for the kind of money that frees us from needing to work for other people. We can quite literally create our own destiny.
And you would throw that away.
I can't even begin to fathom why you would want to live in that world.