Systemd includes many complex things, coreutils includes many simple things. And coreutils are ported to many different OS’es, systemd is linux only. Ask why?
Lets imagine, my linux distro runs with openrc/upstart and I like systemd-journal features. Am I able to run system-journal without any other systemd components running?
I can see you are much more familiar with systemd and thank you for details.
But still I think systemd hardly follow KISS principle.