There are _some_ differences in quality, but I think they're pretty minor.
I recently visited the Philippines and a friend bought some knockoff Crocs at a market. The little rivet thing holding the strap to the shoe had the logo printed right on the exposed plastic, and after a few days, the logo was mostly scratched off. But my wife's Crocs that she bought a decade ago still look fine because there's some sort of sealant on top of the logo.
Is that worth an 800% markup? Probably not. But the knockoffs do cut some corners that the genuine articles don't.