Modern high speed processors have another problem to deal with as well, electromagnetic interference.
Every conductor has some inductance rating, even if it's in the range of nano or pico henries. While that may not sound like much, it does result in the generation of electromagnetic fields around those conductors as high speed signals run through them, and as internal traces on processors get smaller and smaller (the latest Intel i processors are built using 32nanometer traces), these miniscule electromagnetic fields can generate currents in neighboring paths, thus requiring the processors to run on increasingly lower and lower voltages to keep these fields small enough to mitigate such interference.