Because software is information. It is closer to a scientific paper than a loaf of bread, and I do expect those to be free. I do not expect scientists to work for free, but the marginal cost of copying their output is 0 and the social benefit is huge.
Free software, like open science, clearly has something going for it pragmatically. The developer hours put into it have paid for themselves magnitudes of times over. Megacorps hire people to work on free software. If you can't see the value, that's a you problem.
Free software is so important to society that I believe the most reasonable solution is to provide for all people without their need to work for survival. Automate as much as possible such that work is not compulsory, and enough people simply want something to do (and possibly additional pay depending on how the system is arranged) that everything that needs to get done by people does get done.
For now that is fiction, but so is "if all software was free". I do think though that both would lead to a faster rate of innovation in society versus one where critical information is withheld from society to pay someone's rent and food bills.
Most software is free and makes no money - and that has always been the case. There are some very popular and widely-used non-free systems, but most software isn't that, and its developers still pay the bills.
This is somewhat analogous to music or books/literature. Most composers and performers and authors make no money from people copying and sharing their works. Some pay the bills working professionally for entities who want their product enough to pay for it; some do other things in life. Some indeed give up their work on music because they can't afford to not do more gainful work. And still, neither music nor books go away as copying them gets closer to being free.
If my current employer can't make any money from the code we write, then it would collapse faster than a soufflé taken out of the oven too early, and I would be out of a job
It will be interesting to see if this is the case in the long run, assuming "huge" has a positive connotation in your post, of course.
If AGI comes to pass and it winds up being a net negative for humanity, then the ethics of any practice which involves freely distributing information that can be endlessly copied for very little cost must be reevaluated.
Increasingly, I am not putting much weight in any predictions about whether this will happen in the way we think it will, or what it could possibly mean. We might as well be talking about the rapture.
Free software, like open science, clearly has something going for it pragmatically. The developer hours put into it have paid for themselves magnitudes of times over. Megacorps hire people to work on free software. If you can't see the value, that's a you problem.