Difference is that with fiber you can put more data on same piece of glass or plastic or whatever just by swapping the parts at the end. And those are relatively small part of the cost. Most which is just getting the thing in place.
With GPUs and CPUs. You need to replace entire thing. And now they are the most expensive part of the system.
Other option is doing more with same computing power, but we have utterly failed with that in general...
