Yes, but what I'm saying is that per the building code, you need insulation. You also need electrical (but obviously that can be face-mounted). Perhaps some of the interior walls can be painted concrete, but the exterior walls will all have to be studded and drywalled (in most areas). Those studs can also be structural. So you are basically installing 2 structural walls instead of one.

All of the "savings" that these houses claim to have can easily be done with standard, current construction practices; you just need the lead time and planning to do it. Which is exactly why no one does it.