The problem is that those modules are packaged by the developers as opt-out rather than opt-in. It’s a variation on Microsoft’s old embrace-extend-extinguish playbook, only the “extinguish” part hasn’t worked so well because there are some stubborn distros whose needs don’t align with what systemd provides and have maintainers that go out of their way to provide alternatives.

(By contrast, although we may joke about emacs, it’s the myriad of third-party extensions that cause it to just about be its own operating system—it doesn’t all ship with the core.)