This runtime awareness thing is a bit more complicated as just an OS call.
Yes, you can get the pathnames from the OS, but once you have it, what do you do with it?
The "current runtime" is only valid at the time you load the file. Once you load another file from another runtime, the "current" one has changed. Now, what do you do when you want to save a file?
If you store a CR2 or PZ3 or other poser file's content with a full pathname, it becomes tied to the actual location of the current setup. So if you move the runtime, or if you access the runtime from another machine, it becomes a problem.  There has been an ongoing debate whether poser should store relative or absolute pathnames - both have advantages and disadvantages.  Absolute pathnames makes the runtimes miuch more restricted and would break most of the current content (which mostly stores relative paths) and relative paths create problems as described in this thread and has problems with identical named files across runtimes.

SmithMicro has chosen to be on the safe side and not break existing workflows and stayed as it was.

There are solutions to this problem, but they are not simple.