They’re not all identical in features and function though. Nix is different from Gentoo which is different from RPM. And they’re all going to have drawbacks and in some cases, have complete showstoppers.

• Portage/Aur: Not everyone is gonna compile things and if you say use the pre-built options, then this isn’t the right choice.
• Debian/RPM: You’ll never get distro’s to agree to release names or contents, like glibc and ssl versions
• Nix: Learning curve is murder. Not every app is made to be reproducible.
• FlatPak/Snap/AppImage: Loses almost all the advantages of a distro that we take for granted: CVE patching, tested updates, etc.

This is a brief, maybe even unfair overview but it’s not as easy as “just pick one”.

And this ignores the huge pantheon of “language package managers” like pip, gem, npm, cargo, cpan, maven, etc^infinity. Ideally these would just be build dep managers but you get a lot of apps packaged and distributed this way too. Some distro’s/package systems bravely try to keep up but it’s a losing battle.