Thought Experiment: Applying a high frequency high voltage to methane
To decompose methane molecule you have to apply energy about several electron-volts directly to molecule. Since microwave radiation wavelength measured in millimeters, to really decompose methane you'll need ridiculous voltage, much more them mere kilovolts needed to ignite electrical discharge.

And btw, electrical discharge is the easiest way to supply the energy needed for molecule to break.