Why "Data File(s) Used Size (KB)" PerfMon counter doesn't exist?
While it probably doesn't really answer your question I'll post it anyway, it may be more of a comment though.

I don't think anybody outside of Microsoft can give us a definitive reason why this counter was never added.

There is only minor feedback on this connect item asking for a similar thing, but it hasn't been closed as wontfix either:

"Percent Data Used" Performance Counter - by AussieJohn in which Microsoft's last suggestion is:


  Bill: thanks for the suggestion. sp_spaceused provides this
  information and much more. Do you want to expose all or a subset of
  the following info in perfmon?
  
  
  space reserved
  space allocated
  unused space that is reseved but not used
  space allocation between index and data
  


If you really want to expose this information through perfmon you could look at user settable counters as suggested by Marian in this answer