> They're an end-run around the underlying version control system
I assume by "underlying version control system" you mean apt, rpm, homebrew and friends? They don't solve this problem either. Nobody in the opensource world is auditing code for you. Compromised xz still made it into apt. Who knows how many other packages are compromised in a similar way?
Also, apt and friends don't solve the problem that npm, cargo, pip and so on solve. I'm writing some software. I want to depend on some package X at version Y (eg numpy, serde, react, whatever). I want to use that package, at that version, on all supported platforms. Debian. Ubuntu. Redhat. MacOS. And so on. Try and do that using the system package manager and you're in a world of hurt. "Oh, your system only has official packages for SDL2, not SDL3. Maybe move your entire computer to an unustable branch of ubuntu to fix it?" / "Yeah, we don't have that python package in homebrew. Maybe you could add it and maintain it yourself?" / "New ticket: I'm trying to run your software in gentoo, but it only has an earlier version of dependency Y."
Hell. Utter hell.
> I assume by "underlying version control system" you mean apt, rpm, homebrew and friends
No. Git.
No, other trusted repositories are legitimately better because the maintainers built the software themselves. They don't purely rely on binaries from the original developer.
It's not perfect and bad things still make it through, but just look at your example - XZ. This never made it into Debian stable repositories and it was caught remarkably quickly. Meanwhile, we have NPM vulnerability after vulnerability.