Allocated-but-unavailable is a totally reasonable part of the memory hierarchy.

Main Memory => zswap (compressed memory) => swap

In this case, the pages may be logically allocated or not -- the assurance is that the data will be the value you expect it to be when it becomes resident.

Should those pages be uninitialized, the "Swapped" state is really just "Remember that this thing was all zeros."

We could do computing your way, but it'd be phenomenally more expensive. I know this because every thing we introduce to the hierarchy in practice makes computing phenomenally less expensive.