Because you can live in 1 house and rent out other 5 for a lot of stress free passive income. Plus houses are a very safe investment, the price will go up over time.

There is no reason to sell unless you need cash for something else (startup, for example).

What would you do with cash after selling those extra 5 houses? Put it into stock market? Right now when it's mega inflated from cheap money? Too risky, safer to keep those houses.