It makes sense why links can reach higher speeds than CPU cores, but it's not enough explanation for how symbol frequencies got 25x faster while CPU frequencies got 2x faster.
The simple answer to this is that signaling rates were far behind the Pareto frontier of where they could be, and CPU clock rates are pretty much there. CPU clocks are also heat-limited far more than I/O data rates. CPUs are big and burn a lot of power when you clock them faster, while I/O circuits are comparatively small.
Transmitting and receiving high-speed data is actually mostly an analog circuits problem, and the circuits involved are very different than those in doubling CPU speed.
