As an example of the role of distros, consider the Linux Kernel. It used to be common for folks to take the Linus kernel and build it on their own machine and use it in place of their distro's kernel. It wasn't too hard. Linus went to some trouble to make sure a release was clean. I infer that things have changed. All distros take a raw release and fix it up before shipping it. And you want those fixes. It's not impossible to build a Linus kernel and use it but it is probably not worthwhile.
Define: all distros :-). Fedora keeps updating to the latest mainline (stable) and is quite aggressive in its upgrade schedule. RHEL, SLES (and other enterprise distros) not so much. RHEL and SLES have different stability models (which require them to backport a lot of patches). Canonical sticks to one release for its entire release life (which has them somewhere in between). The kernel is actually a really terrible example of flatpaks because it doesn't have dependencies like other packages. Dhaval