There is demand for the all metal type caps, and i suppose people like them. Though I'm unsure fully why or if people think the metal parts are doing the sealing. The seal mainly comes down to the gaskets & o-rings mainly, and the quality of thread engagement regardless of what its made from - could be bronze. But its easier to form consistent bur free non overly sharp threads in nylon than steel and aluminum. And nylon seals against plastic, composite and nylon easier than metal does.
People will unscrew the composite cap on their overflow tank that's been leak free for 150k miles and look for a steel and aluminum adapter to thread onto the same tank to test it for 5 mins out of preference for unknown reasons to me.
You also can't get them as tight, as they are round you just turn them. If seal IS an issue for you, the set that has recessed dimples in the top of caps and a wrench that fits it, I'd imagine you'd go that route.
If you are cross threading caps often enough to damage adapters, you'd prefer the adapter to take the hit than use metal to do so on the customers car.
We've had the opportunity to offer them a few times, just haven't found a reason to charge 1.5-2X to our customers on caps that hold vacuum the same 24+ hours as our current ones (the good metal ones, there's poor all metal sets that hold vacuum for about 15 mins)
