I also want to point out that there's an emotional aspect to this.
Lots of open source is done as a passion project, and passion is a notoriously fickle beast. Things that inspire people to work on passion projects - a sense of ownership, velocity (no need for meetings and jira et al when it's just you. 100% efficient communication!), "shipping"/accomplishment, pride and respect - are all going to be really vulnerable to having to share headspace and ownership with someone else.
I'm personally feeling this. I have a project I'm working on for my homelab that I think other homelabbers, and maybe even some SMBs, will be interested. And I want to give back to the community / have no qualms about someone "stealing" my code. I have an AGPLv3 sticker on my laptop. But the main thing giving me pause is - "but what if people are like 'neat, now please do this feature next!'". Then I'm thinking about it and worrying about disappointing users. And plus, we know that "please do this feature next" is absolutely the *nicest* way that that request will ever be phrased on the internet.
I honestly would feel better if github had a way to let me say:
1. Here's the code, but please don't fork it (but leave the fork button functional. I don't want to actually stop anyone. I just want them to know what I won't be super thrilled.)
2. Please only open issues or PRs for small fixes. An extra if-statement, etc. Please don't add features or redesign anything or "fix my code" at a cosmetic or architecture level.
I know this sounds kinda whiny and entitled and anti-free-software, but I do think it's real and respectful from a human perspective. Locke said that anything you put effort into, you feel ownership of. And by that logic, if someone puts effort into something you currently own "100%" of, they're going to feel some sort of ownership too and you'll feel some part of loss of ownership.