Don't blame the big billboard companies for the barrier to entry in understanding the applicable laws to erect a billboard -- that's the fault of the U.S. government. The Highway Beautification Act was a federal invention, but the good news is that, without that law, there would be no value in billboards as the supply would be unlimited. The state governments then made it even more complicated by heaping their own regulations on top of the federal, so that even the inspectors get confused, And then the local authorities heaped more on top of that.
Once you have a mastery of the system, it's not that hard. But like a video game, until you understand what you're allowed to do and not do, it's very frustrating. But, again, this barrier is what creates all the value.