Don't think "SSD slot". The controller is on the M1 SoC (or the T2 for the Mac Pro). It needs raw access to the flash chips. A better analogy would be if NVMe

Don't think "SSD slot". The controller is on the M1 SoC (or the T2 for the Mac Pro). It needs raw access to the flash chips. A better analogy would be if NVMe drives came with ZIF sockets for the flash chips so there was only one board part number regardless of capacity. The flash chips don't have controllers or any logic on them, they're just flash chips. The manufacturer programs the controller with how many chips there are and the striping, wear leveling, etc at the factory. Now imagine someone says "I'm going to plug a different NVMe drive into the ZIF socket on the first NVMe drive!" That isn't going to go well. Or imagine you add more chips to empty ZIF sockets. The data is already striped and laid out for the original number of chips. To change the layout you'd need to reprogram the controller at minimum.