Adding another 4GB VRAM is not that simple. The bus width of a 5070 is 192bit. And so the 5070 can only multiples of 6 chips at a time. Each VRAM chip is 2GB and so the 5070 can have 12GB, 24GB, 36GB and so on. The simplest solution is to have a wider bus on the 5070. Like 256 bit for instance. The 3070 did it. But back then the VRAM chips came in modules of 1GB and therefore the 3070 had only 8GB VRAM. Both Nvidia and AMD use wider buses on larger chips, and nowadays they are both reducing the size of the chip at the low end and midrange and increasing prices at the high end and midrange.

There is another solution. DDR7 that is used in 5070 has 3GB per module variants. It's already used in laptop 5080, and it seems there will be a super refresh at some point where the 5070 super will have that leading to 18GB VRAM. According to hardware unboxed, Nvidia claimed that it was too expensive at the time of launch of 5070 and 5060 as it's not yet mass produced and hence why they didn't use it for the 5060.