Are you using Deep Search?  Poser should be able to find texture with no problems.  Wordpad works great to edit most prop files or even characters.  One reason resources are not stored in one folder, those same resources might be used in any number of scene files.  Do you want multiple copies of the same geometry files scattered all over your computer?  There ARE software packages that store all of the data WITHIN the scene file.  Geometry is all numbers, right?  That would make each scene file much bigger than it needs to be.

First thing is to decide on a system, and then USE the system. 

PS: confirm you have Deep File Search selected in your preferences.

Best Wishes,
Rªnce