Because they were not intended for customer use and were developed for in house testing of AutoCAD. In some cases they are nice to know for a customer but they were not tested or designed for use outside or even supported so we leave them undocumented. Since these limited number of undocumented system variables of system variables were not designed for customer usage or tested and can be removed in a future release or API change it would be irresponsible to document them. I suppose some might say it would be easy to document them and test but in reality it is not takes localization, testing and some have dependencies or could have 3rd party conflicts causing someone to pull their hair out. So some outside of Autodesk might choose to document them with a disclaimer but you wont find Autodesk documenting them.