Here in San Mateo County we have witnessed fewer people selling their homes over the past 20 years. There are always many explanations for it; people not moving when they retire due to millennials bouncing back or fewer better job opportunities outside of the Bay Area, etc. It appears though that this phenomenon is not limited to the Bay Area.
According to the Federal Reserve, between 1991-2011 the gross flows of people across state lines has dropped by 50%.
In the early 1990s, about 3% of Americans moved between states each year. Today, that rate has fallen by half. There are various theories for this change, but bottom line is if people are not moving, then homes do not need to be sold.