Actually a few years back there was this idea called "White Spaces" that was exactly that... If the TV airwaves were not in use in your area, you could use them for other purposes.
Here's what happened.
Out on the west coast, particularly in Los Angeles, there are no white spaces. In LA you can often get nearly 100 channels with just an indoor antenna. It is unlike any other TV market in the U.S.
Well because there were not white spaces there, they managed to delay the introduction of "White Space" hardware and also the spectrum database needed to support this service.
Then, of course, the cell phone companies came in to buy up a lot of spectrum. Because things are changing, the white space market got stunted. Now that the cell phone companies own the spectrum, there are no white spaces, or at least, much less of them than they used to be.
It's a generally forgotten story that the spectrum situation on the west coast is entirely different from the rest of the country because of the "ring of fire" topography that focuses settlement along the coast.
Out in the Northeast we find that the population is dispersed enough that there are not many transmitters near most sites, but that cities are close enough that there are not many transmitters near most cities. For instance, in Boston you have to worry about interference with Hartford and Portland, ME.
Don't get me started about 900 MHz, which is a non-starter because of the concentration of the electronics industry in Palo Alto, where 900 MHz is crammed with half-baked gadgets made by Stanford kids.