I've noticed that when I download models from Sketchfab, they come as ZIP files but the problem is using the built-in Mac app "Archive Utility."
If a Mac user simply double clicks on the zip file after it is downloaded, they will go into a endless loop of ZIP files inside ZIP files inside ZIP files. I don't know why, but I figured out a kind of clunky solution.
For Mac users, go the App Store and use the free utility "The Unarchiver" instead. It doesn't seem to have the same issues with ZIP files from here as Apple's "Archive Utility" does. It will unpack the ZIP file properly.
I thought this might be of some interest as another person just contacted me in the last half hour to ask me the same question.