Why. Because while your home went up, so did everyone else's, you still need a place to live so if you sold your home you could only buy another home that was of equal value to your current home thus making it is net transfer not a gain...
For instance people moving from the Bay Area to Austin, Portland, and Boise.
Pretty tempting to do just that when your house value doubles in 5 years and you're left with six figure equity with near zero interest rates, in an investment market that is manipulated to avoid any serious correction or recession.
Personally, I prefer to avoid debt, but the past ~20 years has been a perfect environment to maximize leverage in home equity and invest it into more real estate, or the stock market.
The market is fantastic for investors though. A property portfolio delivering 30% growth in a couple of years whilst also grabbing sky high rents or AirBnB sure looks tempting. It's a good time to start a disruptive turnkey property management business.
What they don't realize is that the 'step above' them is getting way out their ability to move into, and they're getting stranded in the tranche they are in.
It's a big new problem with globalism that needs to be addressed.