Yes, I've found AM radio to be something of the Wild West. There are a few local stations I've caught simulcasts on AM for, but not many that broadcast AM only. One of the last stalwarts, 740 AM WPAQ, a bluegrass and old-time station from Mt. Airy, NC finally got an FM antenna in 2020 after over seven decades of AM-only broadcast.

Shortwave AM broadcast works particularly poorly in urban areas, as interference is caused by (but not limited to) tall buildings, bridges/overpasses, telephone wires, mountains/hills, etc. The impact of each of these structures has to do with the angle of interruption, the power of the transmittal source, and your distance away. (This was frustratingly experienced recently when, while on a road trip in a rural part of the state, the only channel I could catch a big college football game was an AM station directly perpendicular to the telephone wire which crossed the road every few blocks. "He drops back to pass, and he [-----] it! What a play! He really [----] that time! This changes the game!") This can be explained by the math of radio physics, but I will spare you the gritty details!

Additionally, most non-national AM stations will power down at night because radio waves do funny things according to the position of the moon (this phenomenon was responsible for Daytimers and Nighttimers on clear-channel frequencies, but I digress). In my area, the vast majority of the time, most bands will be taken over with regional Mexican music from South American stations that don't abide by the powering up/down rules; this is likely my version of the Arabic stations Phil refers to above. This often leads to "chatter" between stations that don't power down but are on the same frequency.

I have used https://radio-locator.com/ to search out local radio stations; they have both an FM and AM section, and you can get calculated theoretical coverage maps (all-day for FM, and day- and night-time coverage for AM) by visiting the info page of each station. They pull from the FCC records, so it's only as good as the records are, but it's a very valuable starting point.

As to your question, "Are there still people in the world who only have access to radio?" -- maybe, but probably not in the US. I've found that most remote places I visit have fewer and fewer radio stations. Even rural parts of my own state, which ranks among America's top-10 most populous, will have several vacant stations, and on trips to more isolated states (rural Alaska, Montana, and Arizona), I've found areas with only perhaps 5-6 total stations on both FM and AM. Where people are cut off from TV and internet broadcasts, the prospects for radio connection are growing increasingly bleak.

There is the opposite side of your question, is there anywhere in the US unreachable by radio, which ironically does have a definite answer: yes, the NRQZ

Quite likely significantly more than you bargained for and much ado about rather little, but it gives the engineering nerd in me a chance to pontificate