We have not yet seen any mini-ITX platforms with Threadripper support, but ASRock is now doing us one better with it's proprietary Deep mini-iTX form-factor motherboards supporting server-grade AMD EPYC Rome CPUs with up to 64 cores and 128 threads.
https://www.notebookcheck.net/Server-grade-mini-PCs-ASRock-reveals-Deep-mini-ITX-motherboards-with-AMD-EPYC-Rome-CPU-support.506212.0.html
Why would you pay 8k for a processor? For one, you don't have to use a 64 core version. Epyc starts at 8 cores (under $500). And even the 64 core unit doesn't cost anywhere near 8k. It's more like 4k for the single socket version (7702P). I don't see a second socket. Do you? One PCIe expansion slot is what you'd expect with a mini-ITX motherboard. And server motherboards often have very simple external I/O.