Well, the two chips I mentioned (DGX Spark uses the GB-10) are both a SoC, so no motherboard needed there. I don't know if that's the full explanation, but it could be a factor.

The SoC design with unified memory is generally well suited for residential use because it's quite energy-efficient, quiet and small (compared to traditional GPU-powered gaming rigs). Great performance-per-annoyance, so to say.