Underwriting and sponsorship are the briefest of ads. It's because people won't open their wallets, and electricity, facility, and meager salaries add up to money.
The FCC also has to ensure that there is staff to review and enforce licensing, the engineering that goes behind it, and the ongoing management of spectrum, as well as common carrier infrastructure, telephony, etc. None of this stuff comes free.
It's not ClearChannel that's made licensing expensive; an act of Congress allowed networks to own more than their fair share, and then this created a money-making opportunity to conglomerate and share resources, while dumbing down content to suit some MBA's target. The good turf was bought up and subsumed into a pit of trash radio.
Who to blame: Congress, for allowing the conglomeration and monopolizing of the airwaves by too few organizations. They allow this with telcos, wireless carriers, and many other industry segments.
The license expense, however, is real. It does take money to run the FCC. Without it, there'd be chaos. This isn't to defend the FCC, rather, to explain that licensing isn't a trivial, go to the license branch sort of activity. I can bounce a beam off the moon from my back yard as a ham radio operator. But I can't do this: run a pirate station-- without consequences. Spectrum management is non-trivial. There are many competing factions wanting spectrum. On a cable, or via the Internet, there is much "spectrum".