You should design one and Kickstarter it.
You know what? I totally would if I actually wanted one myself.
The thing is - I don't. And I don't think anyone else really wants one either, otherwise the market would already have responded
It would be easy enough to prototype with something like a Gigabyte Brix or ISK300, but it would always come down to an issue of how to power it and that means overpriced SFX PSUs or even more expensive external supplies and converter boards. Gamers are typically after good value for money, because any budget that isn't wasted on size-reduction can be spent on faster GPUs - and PicoPSU's and fancy form-factors don't really fit the "good value for money" bill very well!