Earlier quoted context omitted.
I believe that the issue is that graphic cards require really fast memory. This requires close memory placement (that's why the memory is so close to the core on the board). expandable memory will not be able to provide the required bandwidth here.
The universe used to have hierarchies. Fast memory close, slow memory far. Registers. L1. L2. L3. RAM. Swap. The same thing would make a lot of sense here. Super-fast memory close, with overflow into classic DDR slots. As a footnote, going parallel also helps. 8 sticks of RAM at 1/8 the bandwidth each is the same as one stick of RAM at 8x the bandwidth, if you don't multiplex onto the same traces.
Point of fact: GPUs don't even use all the PCI Express lanes they have available to them! Most GPUs (even top of the line ones like Nvidia's 4090) only use about 8 lanes of bandwidth. This is why some newer GPUs are being offered with M.2 slots so you can add an SSD (https://press.asus.com/news/asus-dual-geforce-rtx-4060-ti-ss... ).