I respectfully disagree. Local stations are hardly competing with the Internet and Sirius XM, who do not subscribe to local books. Moreover, a good chunk of the homogeneity started before Sirius XM became popular.
My point was that there was a trend towards unified national music lists in each genre that goes back to the 50's with Your Hit Parade on TV. Soon we had American Bandstand and pop music entering shows like Ed Sullivan and Johnny Carson. By the time we got to the 70's, radio stations all used airplay lists in publications like Hamilton, Gaving, FMQB and Radio & Records.
By the later 70's we had thousands of stations doing taped formats, ranging from Beautiful Music to Top 40 and Country all using the same playlists done by just a few syndicators. The 90's brought satellite versions of taped formats, and then we got Internet streams and satellite radio.
None of that had to do with consolidation... it had to do with the fact that distribution methods improved, making it possible to form de facto networks of stations in each format with identical playlists.
If people are not exposed to different music and regional variation is not allowed, then playlists do not even have the opportunity to differ. Streams do not necessarily have to dictate local taste. There's no reason why an AC station in Jacksonville should have the same playlist as one in Des Moines.
There is no reason why it should be different, either. It's the same country, the same format, the same culture. This is no different than when Gunsmoke got 30 ratings in every market, from Miami to Medford.
It makes a difference because of local preferences. These stations could be doing better if they did a better job catering to their markets. But they no longer do.
If you do music research in different markets for the same format, you will find that "good" is the same thing, market after market. With very rare and very moderate exceptions, in research the same songs score the same way "everywhere" among partisans of a particular format.
Jocks also build station loyalty and talent also give people a compelling reason to tune in.
In most cases today, listener after listener will tell you that, outside of mornings, they want us to "shut up and play the music". Morning shows are a bit different, as the best use music as fill between bits... and people seem to overwhelmingly like that at the start of their day. But, just as they like personality in the AM, by afternoons they don't want interruptions after a long day.