but that also shows that most modern software is poorly written

Does it? I mean, this is especially annoying with old software, maybe dynamically linked or PHP, or stuff like that. Modern tools (go, rust) don't actually even have this problem. Dependencies are annoying in general, I don't think it's a property of modern software.

Yes, that’s exactly point point. There are many options, yet people stick with Docker and DockerHub (that is everything but open).

Who are these people? There are tons of registries that people use, github has its own, quay.io, etc. You also can simply publish Dockerfiles and people can build themselves. Ofc Docker has the edge because it was the first mainstream tool, and it's still a great choice for single machine deployments, but it's far from the only used. Kubernetes abandoned Docker as default runtime for years, for example... who are you referring to?

Yes… maybe we just need some automation/orchestration tool for that. This is like saying that it’s way too hard to download the rootfs of some distro, unpack it and then use unshare to launch a shell on a isolated namespace… Docker as you said provides a convenient API but it doesn’t mean we can’t do the same for systemd.

But Systemd also uses unshare, chroot, etc. They are at the same level of abstraction. Docker (and container runtimes) are simply specialized tools, while systemd is not. Why wouldn't I use a tool that is meant for this when it's available. I suppose bubblewrap does something similar too (used by Flatpak), and I am sure there are more.

Completely proprietary… like QEMU/libvirt? :P

Right, because organizations generally run QEMU, not VMware, Nutanix and another handful of proprietary platforms... :)