> The idea that your OS distro needs to explicitly support and package every software you could ever want to use seems crazy to me.
No one is under that impression, not maintainers nor users. The idea is that every software that is common and/or popular and/or useful enough to someone who is willing to maintain it, is managed by the distribution/repository staff itself, as they're the ones who knows the distribution best. Some software requires patches to be fully compatible, some distributions expect certain files in certain places and so on, and no one expects every developers to understand the idiosyncrasies of every distribution they want to potentially ship to.
But that doesn't mean ALL software you use has to come from these official software repositories maintained by the distribution. Most of my packages are from the official Arch repositories, maintained by people who know what they're doing extremely well, and almost gives you certain "guarantees" by the mere existence in that official registry. But then some software I use is installed via the AUR, which is more like npm, where anyone can publish anything, close review of everything is required for this, it's the wild west. And some software I install/run by having local GitHub checkouts I build binaries from. Finally some are just stray binaries in ~/bin/ that I've had for decades and don't even know where they're from anymore. All are "correct" for what purpose they serve, yet the official Arch repositories still remains extremely important and valuable for stability.
Still, the software that almost all of the distribution users use, like wayland or systemd, is maintained by these people and explicitly "supported" by the distribution, as far as that term holds in FOSS.