There have been many questions over the years as to why PTC only furnishes the limited (and generic) material files for the materials library. I believe it to be many factors, with liability one of the biggest (my opinion).
I have seen a few scenarios at companies I've worked at, supported:
1. The engineers themselves, as they are building their designs, create their own materials as they go along, if the limited ones supplied by PTC are not detailed enough for their needs. The prudent thing to have happen here would be for the engineers, as they create these files, to have a mechanism in place where they get validated/uploaded to the "pro_material_dir" library. Unfortunately, this rarely happens, at least in my experience.
2. A company will invest the time/energy into creating a comprehensive library, whether it be a manual process, outsourced, etc. MatWeb seems to be a popular site to use, but the BIG drawback is the lack of compatibility with ProE (Creo). Most of the major CAD packages have direct translation capability within MatWeb, puzzlingly, not Creo. A manual translation from a CSV file is need, which from what I've seen, is a time consuming process.
3. CAD admins will get "hand me downs" from other CAD admins - peer-to-peer sharing if you will of files. Then there is the question of "format", .mat vs. .mtl.
I recently rediscovered a fairly large set of material files that I created/acquired back in 2010, and updated in 2013. The data is the files was derived from the Roark & Young Handbook. Although I feel very confident in the accuracy, I am human, I make mistakes, and as such make no guarantees. if I were a CAD admin getting these files to use for my company, I would definitely look a them for accuracy.
In closing, PTC will never offer a material files library with their OOTB load, so we are left to the way we have always done things. I hope these files help you, if you so choose to use them. I also would hope that "others" might share their libraries as well. Wouldn't it be great if the PTC Community had a set for all to use?