Well, actually, no, not everyone is free to use alternatives. Anyone using CI for "Trusted Publishing" of packages to PyPI or npm needs to use GitHub Actions or GitLab CI/CD. CircleCI and Travis CI are not supported. So many big open source projects for the two most popular languages in the world are now locked out of the alternatives you propose.
(I find it extremely sketchy from a competition law perspective that Microsoft, as the owner of npm, has implemented a policy banning npm publishers from publishing via competitors to GitHub Actions - a product that Microsoft also owns. But they have; that is the reality right now, whether it's legal or not.)
Trusted Publishing on PyPI supports Google Cloud and ActiveState as well. It’s not tied to GitHub or GitLab. To my recollection I looked at CircleCI support a while back, and ran into limitations on the claims they exposed.
(It can also be extended to arbitrary third party IdPs, although the benefit of that is dependent on usage. But if you have another CI/CD provider that you’d like to integrate into PyPI, you should definitely flag it on the issue tracker.)
I was never convinced that trusted publishing solves any security problem, other than letting pypi eventually solve the problem of banning russian/iranian/whatever people just by relying on github doing it for them.
(I find it extremely sketchy from a competition law perspective that Microsoft, as the owner of npm, has implemented a policy banning npm publishers from publishing via competitors to GitHub Actions - a product that Microsoft also owns. But they have; that is the reality right now, whether it's legal or not.)