We use coax, fiber, and twisted pairs to transmit signals for a reason -- the signal stays inside the cable(mostly) and other signals(interference) is kept out

We use coax, fiber, and twisted pairs to transmit signals for a reason -- the signal stays inside the cable(mostly) and other signals(interference) is kept out of the cable. The line voltage is not the limiting factor. To get gigabit speeds your need to transmit at RF frequencies, and that hundreds of miles-long power cable is one giant antenna that will interfere with other users of the spectrum. The power levels used in home powerline network gear are tiny, and the sources of interference much smaller. And even they have power and interference restrictions from the FCC to protect other users of the spectrum.