If the gates of the transistor are taking up all that space, but FinFETs and newer gate all around FETs have significantly sized voids in 3D space, you just can

If the gates of the transistor are taking up all that space, but FinFETs and newer gate all around FETs have significantly sized voids in 3D space, you just can't practically use those voids for anything else without cutting into yields or having awful leakage issues.