I may be wrong on this, but here goes:
The Highway Beautification Act only regulates billboards within 660 feet of the highway right of way. It also says you can only have billboards in areas zoned commercial or industrial. So say you have a piece of farm land zoned agricultural, residential, etc. And say your local municipality has very lax zoning laws, or this is in 1975 or so before municipalities really clamped down on billboards. If you set the billboard more than 660' away from the highway, it does not fall under the HBA requirements. But it needs to be gigantic so drivers can see it from the highway since it's so far away. Hence the giant phone pole signs far away from the highway. I think most of them were built 25+ years ago.
I've noticed that CBS owns a lot of these signs in Ohio and New York. They are often falling-down pieces of junk. One huge windstorm and they're done. Some are even painted the old-school way. Even with their large size some are of questionable value; the driver might miss it unless they know it's there.