While I agree they look cool, they're an engineering nightmare and absolutely require titanium to build and that shit is too expensive for very little benefit.
The point of a variable wing was to increase the effective flight envelope and there's no point now that the same thing can be done with modern avionics and materials.
Except the US doesn't have a cheap, easily available source of titanium.
The stuff we used for the SR-71 and F-14 had to be gotten surreptitiously from the Russians.
That's why the Space Shuttle didn't have the titanium heat shield it was designed with and had to rely on the newly invented, much more delicate, ceramic heat shields. Which, it can be argued, resulted in the all of the deaths of the Challenger crew.