You _can_ pay a microsolder shop to put double capacity ram chips of the same type on your gpu but that is risky.

as far as sticking ddr5 onto a board you'd need an entirely new memory controller to handle that, and the translation would be expensive in terms of latency. Those cards are already basically at capacity anyway as far as the weight that they can handle and density. They are multilayer boards already. You'd need some kind of header on the top to go in an empty bay, like an NVLINK or something if they still have that.

They just choose not to put larger gddr5/6/6x chips on them because they know that you will buy a bigger card.