I mean, there are solutions, but none of them seems to have a large enough mindshare and efficiency. (Even though Github's code review tools are pretty spartan.)
They can be. A PR can be made and code review conducted by submitting a patch to a mailing list. That's how the kernel and, I think, git itself is developed.
CI/CD is really a methodology. It just means integrating/deploying stuff as soon as its ready. So you just need maintainers to be able to run the test suite and deploy, which seems like a really basic thing.
PRs and code review are not. CI/CD is not.
I mean, there are solutions, but none of them seems to have a large enough mindshare and efficiency. (Even though Github's code review tools are pretty spartan.)