I feel like there's just not a significant market for that.
Plenty of things that want that small size don't need the better processing, and relatively few things that need that better processing are concerned about space.
You can always built a micro ITX real server with some of the AsRock server boards. Not as small as a NUC but pretty small considering their capabilities.
Have you looked at the size of the latest videocards? That's just the GPU (and secondary components), and it's bigger than a NUC. The power supply needed to feed a powerful GPU and CPU is larger than a NUC. There's a lot of empty space inside a modern tower case, but you're not shrinking high end components down into a NUC. It's just not feasible.
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