Even if the cost were the same of the RAM itself, you'd need much bigger and more expensive CPU to deal with it. Running 17 chrome tabs doesn't benefit at all

Even if the cost were the same of the RAM itself, you'd need much bigger and more expensive CPU to deal with it. Running 17 chrome tabs doesn't benefit at all from that HBM and all the additional hardware+software complexities that come with it. You want a specialized coprocessor to handle specialized workloads. The GPU exists separately from the CPU for a reason.