Radio in a spot

I've had radio on my mind. Why is it such a difficult medium to write for? From time immemorial, mankind has been receptive to aural stimulation. Why then, is radio such a neglected medium? Why do we see so much bad radio? Listening is, arguably, a better way to remember things. Why then do advertising agencies pay so much more attention to print and so little to radio? Television is, arguably, the most expensive medium to advertise on. Why then do advertising agencies pay so much more for television and so little attention to an immensely cost-effective, audio/visual medium like radio? And yes, I do believe radio is an audio/visual medium.