Because the term 'unix system resources' is made up after the fact. It is not in the FHS. There are no sources for it. It is hearsay.

There is plenty of evidence that /usr used to be the home directories, iirc it was like that in Illumos until 1988.

Edit: Evidence for "user": https://www.youtube.com/watch?v=tc4ROCJYbm0#t=13m40s