Maybe because that is a VERY different problem than the one discussed here. Building a single analog chip with 1 billion neurons would cost billions of dollars

Maybe because that is a VERY different problem than the one discussed here. Building a single analog chip with 1 billion neurons would cost billions of dollars in a best case scenario. A Nvidia card with 1 billion digital neurons is in the hundreds of dollars of range. Those costs could come down eventually, but at that point CUDA may be long gone.