I am flattered that Magic asked for my humble opinion. In seven years of doing "localized" radio, I made a lot of connections in the business and had job interviews with various "bigger" radio stations including some classic rock ones.
Here is what I can offer, please understand this isn't necessarily the way that ALL rock radio stations do business. However this is a general blueprint that is used by a number of stations.
What you see below is a couple different business models for AM/FM stations (I didn't elaborate on satellite radio). Some stations use one, some use the other one and some stations use a combination of both.
1.
Outsourcing
Some radio stations actually outsource the bulk of their daily playlists to outside companies (in the music business) who make money by creating playlists for a large number of radio stations, depending upon their format. In the "old days" before things became too commercialized in the business, these stations would leave the playlist responsibilities to the music director or program director, depending on their staffing situation. That's how it used to be. Some stations (particularly smaller ones) still use their internal staff. Anyway, these outside companies generally rely on a selection of songs that are either "popular" (newer stuff) or have a long track record of success on the radio and are deemed safe (i.e. songs by Zeppelin, Skynyrd or Aerosmith). Basically anything that the company feels will "appeal" to as many people as possible, so the station has somebody tuning in to their advertising during commercials.
2.
Universal "Canned" Playlists
-As some of you know, a lot of radio stations (AM/FM) are now owned by various corporations who also own a large number of other stations either regionally or on a national level. Let's for example say a corporation is named "Soot and Stars." Let's pretend "Soot and Stars" owns 35 rock radio stations in the United States, ranging from Maine to California and all points in between. Assuming all of those radio stations are the same music format (rock), chances are they will
ALL be using the
very same playlists or playlists which are very close to one another with very slight variations. They will likely be playing the exact same songs as one another at around the same time of the day. Doesn't matter whether the playlists are put together in house (somewhere at the corporation) or if they are outsourced like I mentioned above.
See, the logic here is that since all of these stations are broadcasting to different parts of the country to different audiences, the assumption is that these listeners won't notice the difference anyway. Naturally, some of these radio stations will overlap meaning a listener will be able to hear two of the stations playing almost the exact same list of songs. So those of you who are in an area where a couple of different rock stations play the very same songs, that is what is going on. Besides, if the Soot and Stars corporation owns 35 rock radio stations, it is a hell of a lot less expensive to "produce" one basic playlist that can be used by 35 stations than it is to produce 35 different playlists for all of these different stations.
Finally, going back to #1, sometimes you will have two different rock stations who are owned by two different corporations but perhaps they are outsourcing their playlists to the same group and even though these two stations in your area are owned by different companies, they outsource their work to the same firm and thus will have very similar playlists. Got it?
With that said, this isn't how ALL rock stations do things but there are plenty of stations who use either one or both of the above blueprints to come up with their playlists. Again, this is based on friends I have in the industry and some of my own job interviews while learning about the station or corporation I was applying for.
Same formula also applies for pop stations and country stations, even classical music stations.
I guess the Scorpions stuck around for over 4 decades and have sold 75 million albums or so on the strength of two songs alone. 
