Beside stacking layers on chips, stacking multiple dies is also used to increase the density per PCB area. (I believe that works for DRAM and flash.) The packag

Beside stacking layers on chips, stacking multiple dies is also used to increase the density per PCB area. (I believe that works for DRAM and flash.) The package you see on the PCB may actually hold tens of silicon slices which themselves each hold tens of layers. This method can be used until heat dissipation becomes an issue, which may be the reason why CPUs/GPUs are not stacked.