Junk metal that isn't round for the balls and the other surfaces not being machined smooth for the balls to rotate on.
I have a pair of Gojacks and they work fabulous. I bought the pair very reasonable at an auction and had to replace a couple of casters. The casters cost me around $70 for a pair but they are worth it for as good as they work. I would love to find a second pair at an auction but it hasn't happened yet.
If you look at the Gojack picture you will see that the wheel isn't offset from the center pivot as much as the casters you posted.
