Why don't plants use radio waves?
Even if the atmosphere is mostly transparent in the radio domain, most of sunlight is visible light.

Also, to make photosynthesis work, you need photons of high energy (say: wavelength shorter than 700nm) in order to excite electrons in the reaction centres. Radio waves cannot do that.

The most widespread photosynthetic organisms make use of visible light and water, which are plentiful on Earth.