Best i can tell, this has come to be because XDG/Freedesktop stopped being about specs and became all about code.

Rather than cook up a protocol spec between high level and low level, they write a -kit/-d (more and more often these days as part of the systemd shoggoth) and makes its APIs (that are subject to change from version to version) the "protocol spec".

And most of this gets prototyped over at Fedora, by RH employees...