Comment by entropicgravity

It's because energy created by the transmitter must degrade as one over R squared in the far field. The frequency (or wavelength, have your pick) has nothing to do with the energy transmitted because energy must be conserved. Putting in the frequency term then violates conservation of energy between the antennas. Then, at the receiving antenna the error of conservation of energy is then patched up by assigning a bogus 'gain' at the receiver. The transmitter and receiver are asymmetric but the path loss equation pretends that they are because that's easier for most people to understand and it works out 'end to end'.