Because wasm virtual machine has contiguous linear memory. So if you do this, you're no longer running wasm.

Besides, why would you design around the lack of a MMU? Even in embedded space it's often there.

No I mean what if we updated the wasm spec to allow it?

But why would we do that when the hardware already has everything that's needed to present the app with a flat contiguous memory space? Why push unnecessary complexity at every single wasm app out there?