The simple answer is that most PC manufactures don't manufaure the chips used in it's design. They rather select what chips the chip industry has avalible. So the question really morphs to why do chip manuafacures use the voltages the do offer rather then much higher voltages. The answer seems to be that as chips have become faster and more dense in transistor counts the physics favor moving to lower and lower chip voltages, where today we see that the internal Intel processor chips work at around 1 volt level internally to best deal with heat dissapation and clock speed used. So the answer to the electrician in your class is to tell him that wire or chip conductor size has not been a limit, but rather the internal heat dissapation by the transistors the make up the chip design, packing millions and millions of transistors onto a single small die creates very difficult heat management problems that can only be dealt with by using lower and lower switching voltages.
Lefty