Nothing stops a user from packaging and installing these things in KISS, they just aren't included by default.
I envision KISS as a minimal base in which you extend to suit your needs and not something you need to cut to size.
As I state in the Philosophy; "it's easier to add things to a system than it is to remove them".
A user has actually gone ahead and done this! \[1\] They run KISS with systemd, pam, dbus, pulseaudio, glibc, chrome etc etc.