Out of those 1000 buyers, how many will seriously put in offers AND be qualified during the period that those same 21 listings are on the market? Not saying this is an ideal situation, but you have to keep in mind that not everyone buys at the same time.
Some may even go elsewhere for investments since the East Coast and parts of the South are in a similar pickle now as SD was two years ago. (And it’s a lot easier to get a decent cap rate on 1-3 families there than in SD.)