UniChris

Presumably because the sideplates of the higher speed count chains are too thin.

In the re-usable links that I'm familiar with, the capture of the pin is done with a stepped hole in the side plate - the plates have to move in to release it, and normally the installed chain end keeps them apart. It's a 3d feature.

If I had to guess, in the thinner sideplate chains the capture is more of a 2d feature than a 3d one, and involved essentially munging the metal to let the pin pop through a squeeze between two partially overlapping holes of different size, and not back out. Since the metal would actually flow it wouldn't work very many times.

Another way of looking at it is that a 7 or 8 (or maybe 9?) speed quick link won't securely lock unless installed on a chain (which sometimes leads people to think real ones are defective or fake, though of course fake ones are rampant).

Pure speculation as I don't have one to look at, but I wouldn't be surprised if a 10, 11, or 12 speed one would lock all by itself (but then, if you try it, you've used up its official life...)