Before we had M2/U2 NVMe drives, there were quite a few manufacturers making drives that plug straight into the PCIe bus where they could easily access the full 16 lanes, and they still can...
Intel seems to be the only ones making PCIe drives (Octane's are funny; they don't look fast only 2000 MB/s, although they do win on latency and random access). Anyway, there doesn't seem to be much demand for that speed, I guess it's a case that x4 U2/M2 are good enough.
Speaking of which.. I'm am surprised there isn't a similar network interface utilising PCIe with a low overhead protocol the same as NVMe does. Getting towards the point where a small number of machines could share memory and CPU's between machines. I know we have Infiniband but that has always come at quite a cost, mainly in supercomputers.