AM stations, unlike FMs, can be filed for if an applicant feels that there is a "space" for them that fits technically. Daytime AM applications are excluded, although today I doubt anyone would want one.
The extended band (Above 1600) was intended for 10 kw day and 1 kw night operation. As such the band is totally full, although a few that could not succeed are silent. The coverage of those stations makes them limited to small areas (250 watts on 550 would cover more than 10 KW on the extended band in the daytime (when it matters) .
Actually, the rest of the band in most of the populated areas of the US is totally full based on the interference and protection rules. Getting a fulltime license for a new AM usually requires a directional antenna system, a big piece of land and a lot of money.
The clear channel stations date back to the early 30's. The good regional channel stations date to that era, with few good ones coming on since WW II. Remember, to operate on a regional channel at night you might have to protect a station licensed long ago that might be over a thousand miles away! For example, the reason 620 in Tampa and Milwaukee are directional at night is to protect each other.
But mostly, nearly nobody wants an AM station today. There are plenty of existing ones for sale with no buyer, and in most cases those that are surviving are being milked by the owner while the band is still viable.
If you want to waste money, buy a used Yugo. Much better deal than a dog AM.