It's part of the systemd project umbrella? It's part of the systemd monorepo? It uses libsystemd?
Maybe it's simply easier to maintain this way?
elogind exists, if you care. It exposes the DBus interfaces that logind supports for applications to call.
Thus, environments like Gnome can be supported on non-systemd systems if they emulate and/or expose and implement the required DBus interfaces.