And I still don’t know what’s the point in separating /bin, /sbin, /usr/bin and /usr/sbin.

This goes back to the olden days when disk space was measured in kilo and megabytes. /sbin/ and /usr/sbin have the files needed to start a bare bone Unix/Linux system, so that you could boot from a 800kb floppy and mount all other directories via network or other storage devices as needed.

It gets even more complicated nowadays because most DE will mount removable drives somewhere in folders like /run/$USER/

Mostly historical reasons, /home was often a network mounted directory, but /root must be local.

And only regular users have their home in /home

Well then the answer will most likely be: because they can and want to do it.

Maybe because that person uses systemd everywhere else and just doesn’t want the overhead of maintaining two different init systems.

There is no indication that anyone will be pruning systemd from distros in the near or far future. Systemd is here to stay and if anything it will only spread into more and more places as can be seen with projects like this.