Two reasons. One, because people vote, not real estate. And two, your map has a fraction of the counties that went for Biden. Biden took 14 counties in Wisconsin, not 2. He took 9 counties in Maine, not 0. He took 14 counties in New Mexico, not 1. And on, and on.
The map may be misleading but Biden won 477 counties.
https://fortune.com/2020/11/12/joe-biden-voters-election-results-by-county-us-gdp-democrats/
There are 3007 counties in the US.
That’s 15.9% of the counties