They're not doubleshot the same way.
The entire "top" of the keycap is one piece of PBT (so is the stem). It's dyesublimated with the legend (or rather everythign but the legend is dyesubbed). Then an ABS "skirt" is doubleshot around the side. The entire top is one piece of PBT.
As far as why no-one else has that? it's hideously expensive, and TG3 (deck) has the only tooling. There's no profit in spending tens or hundreds of thousands of dollars kitting up a new plant to make keycaps like this.
I will admit, taht they are great keycaps: some of my favourites for sure. I do prefer blank ones (preferably black POM) on my backlit keyboards, so I get a sweet underglow effect. Every keycap is backlightable.