Why is the housing price in the USA not going down even though there are more than 20 million empty homes?

It depends on the market. In Northwest 

 Indiana we have a shortage of available 

homes. There are way more buyers then

 sellers. Which means many homes have 

multiple offers. Many people from Illinois are 

leaving because taxes are so high, which adds to the housing shortage. I guess it depends on your market.

Low supply and high demand

there are couple factors which enforce each other