okay, how are you going to fit it?
You could do it with something like CAMM on the back side of the board, but only if you dropped the GDDR speed dramatically. Nobody would like the crippling performance drop the GDDR bandwidth reduction would bring with it.
And also giving customers easy way to increase VRAM and not making them buy a new product is bad for the bottom line of AMD, NV and Intel.
Yeah, that's not a concern. If it were possible, then Intel or AMD would do it. They're competing for like 2% and 10% of the market, and increased longevity would be massively offset by the piles of money afforded by taking a chunk of Nvidia's pie.
It's not possible because the GPU relies on increasing memory bandwidth to keep up with increasing shader vertex or texture processing.
The most extreme GDDR5 overclocking is at around DDR5 7200 or so. So 7.2Gbit/s. It pumps MCLK at a languid 3.6GHz. Pathetic.
Your GDDR6 gpu is pumping the WCK pin at 8 or 9 GHz to hit 16 or 18Gb/s. GDDR6x uses QAM so it's "only" pushing around 5GHz.
The PCB routing is kept physically as short as possible, and arranging it so the traces are all EXACTLY the same length and impedance is critical. You simply cannot get a signal through a PCB at that rate with a mechanical connector in the way.