Phoenix probably has really bad ground conductivity too, being in a desert.
We do, but that's not the reason our AM stations don't cover the metro. The problem is that the metro, which was about 70K people in 1940, almost all within a 15 mile radius of downtown Phoenix, is now closing in on 5 million within a 60 mile radius of downtown Phoenix.
The only two stations that come even close to covering that much area are KFYI/550 (originally KOY, which moved there from 1390 in 1941) and KTAR/620, which moved there in around 1929, and built their current facilities on what was then farmland in 1938. That farmland has been a mall since the '50s, surrounded by a residential area. We have other stations that run 5 kW at night, but they're directional. Not good these days.
When the 50 kW clears were allocated and built in the '30s, there was no need for any in Phoenix. The metro outgrew the stations beginning in the '50s, when it was too late to allocate a Class A (or 1A, in that era) to the city. Now, they are no longer necessary.
Nothing unusual about that in any market. Look at Chicago. The original Class A facilities of WMAQ/WSCR (Bloomingdale), WGN and WBBM (Elk Grove), WLS (Tinley Park), and WCFL/WMVP (Downers Grove) were out in the middle of nowhere when they were built. Now, each site is valuable to developers who are willing to pay 7 figures for them. All five either have or will move to what really are lesser facilities, probably with almost no reduction in coverage of the Chicago metro area. Being received in 38 states and Canada no longer matters, as long as they still put good signals into the 8 county Chicago/NW Indiana metro. Outside that area? TuneIn and/or the stations' websites are your friends.
But here in Phoenix, there's nothing they can do. Ancient Modulation is pretty much dead in this market anyway.
BTW, I know this is the Miami board, but I haven't visited there in over 50 years. I would have to ass-u-me that South Florida is at least somewhat similar to Phoenix as far as growth is concerned.