The problem with GPUs is they're designed to be saturated.
If you have a CPU and it has however many cores, the amount of memory or memory bandwidth you need to go with that is totally independent, and memory bandwidth is rarely the bottleneck. So you attach a couple memory channels worth of slots on there and people can decide how much memory they want based on whether they intend to have ten thousand browser tabs open or only one thousand. Neither of which will saturate memory bandwidth or depend on how fast the CPU is, so you don't want the amount of memory and the number of CPU cores tied together.
If you have a device for doing matrix multiplications, the amount of RAM you need is going to depend on how big the matrix you want to multiply is, which for AI things is the size of the model. But the bigger the matrix is, the more memory bandwidth and compute units it needs for the same number of tokens/second. So unlike a CPU, there aren't a lot of use cases for matching a small number of compute units with a large amount of memory. It'd be too slow.
Meanwhile the memory isn't all that expensive. For example, right now the spot price for 64GB of GDDR6 is less than $200. Against a $1000 GPU which is fast enough for that much, that's not a big number. Just include it to begin with.
Except that they don't. The high end consumer GPUs are heavy on compute and light on memory. For example, you can get the RTX 4060Ti with 16GB of VRAM. The RTX 4090 has four times as much compute but only 50% more VRAM. There would be plenty of demand for a 4090 that cost $200 more and had four times as much VRAM, only they don't make one because of market segmentation.
Obviously if they don't do that then they're not going to give one you can upgrade. But you don't really want to upgrade just the VRAM anyway, what you want is for the high performance cards to come with that much VRAM to begin with. Which somebody other than Nvidia might soon provide.