This discussion lead me to an idea:
When we simulate a brain, the brain is unreal, not running on the hardware at all. It's represented by something else running on the hardware.
If we made brain like hardware of sufficient complexity so that it could differentiate "itself" from everything else, that closed loop may invoke consciousness. It would be running on the hardware directly.
This, to me, is the one thing conscious organisms have in common. And yes, I know that idea of anything but us being conscious is highly debatable. Let's just say my cat is conscious, and I believe that it is conscious. You, of course, can disagree and that's fine. None of us knows anything, and we are all sharing thoughts and that concept below is mine:
To simplify, consciousness may require a feedback loop. The nervous system, brain and body all are integrated with sufficient complexity and fidelity so as to make it possible for the brain to arrive at a sense of "self", able to differentiate itself from everything else. Self-simulation is different from outside stimulation, and awareness of the body falls out of that for free.
This is why a simulation won't render consciousness. Whatever consciousness ends up being is not a part of the simulation. The very nature of simulation means it's all about things we know and at least understand exist. And since we do not yet understand consciousness, it's not going to rise as an emergent artifact of things we build.
Or... at the very least, we need to build something complex and robust enough for it to be able to differentiate itself from the greater environment it is in before we have a chance at consciousness happening for it.
This all does allow for consciousness to be computable. It's just that the computation needs to be done on a system capable of self-differentiation and awareness.