I think it's also because if you're a company that wants to make money doing real estate related activity, it's easier and more profitable to just buy existing buildings and sit on them than build new ones. This is a recent trend, and home building companies do still exist but...

"Between 2011 and 2017, some of the world’s largest private-equity groups and hedge funds ... spent a combined $36 billion on more than 200,000 homes in ailing markets across the country. In one Atlanta zip code, they bought almost 90 percent of the 7,500 homes sold between January 2011 and June 2012; today, institutional investors own at least one in five single-family rentals in some parts of the metro area"

https://www.theatlantic.com/technology/archive/2019/02/singl...