And how many percent of US homeowners are mortgage-free?
It's unlikely ~100% of the investor owned homes are free and clear and only ~3% of owner occupied homes are owned free and clear.
For one, the tax code HEAVILY incentivizes investors to own property on leverage. For two, the only reason property is/was a good investment was because of artificially low interest rates for most of the last ~25 years (an entire generation of investors). When you combine that with a leveraged bet that cannot be margin called, you will make a FORTUNE (and many did).
Leverage was the way.
I'm willing to bet the majority of free and clear owned homes are owner occupied (simply because 2/3rds of homes are owner occupied).