The correct answer is because Nvidia and AMD were too short-sighted and/or didn't care to design their lower end products with a bus size that would allow more appropriate Vram configurations, and the market has moved on and decided for them.
Can't say I get the objection to 16GB I see bandied about considering the above scenario. It is effectively being against having ample space with room to spare, and instead arguing no it should be halved and be a challenging tight squeeze, I mean why.
Above 8gb should be the standard now . Could say 8gb should be reserved for the very lowest tier
Adding another 8gb for extra $100 to product that's already over priced