Reverse Engineering Stereolithography?

I’ve seen this question posted around the web quite often but I can’t seem to find something that works. I tried exactly what was done here but my FreeCAD stops responding. I’ve tried it both on my laptop and desktop (32Gb RAM+i7-4770K) so I don’t think my computers are lacking the power. It is so easy to convert .step to .stl but not the reverse. What’s the deal??