We already use high voltage for power transmission. Step up transformers at the power plant bring the voltage up to 230kV for transmission and step down transformers within the city bring the voltage down to normal.

My guess for the frequency, is that it is a harder conversion to perform with acceptable loss. Compatibility reasons doesn't allow for just bumping it up. Also let's not forget that AC frequency actually comes from the revolutions per second the power generator does, so there may be some other physical limiting factors here.

Last, some underground high power transmission lines are actually emerged into oil, to keep them cool.