1) Bigger number sells
2) Not everything is about the GPU performance, for some users the 16GB VRAM is actually the point of the card
You could say the exact same thing about the 4060 TI 16GB, you could say why don't they make a 24GB 5070, and all sorts of things. Market segmentation mostly. They don't want people to not buy the 5070 Ti and 5080, so they don't make a high VRAM count card, and they don't want a card that competes directly with the 4090/5090.
Large VRAM cards will also eat into their workstation card sales, which do tend to have the larger memory pools.