I’m going with the “idiot” theory, and adding “snob” to it. I’m guessing he has a flashy car, too. Probably leased.
We have kids and like our privacy, or we would probably do it, too. $600/month is awesome passive income, and you get to supervise your tenant, unlike a rental property.
We do plan to take it to the next level when we buy/build our forever house – a casita with its own entrance, living area, and kitchen/laundry. The MIL will live there for a while, but once she’s gone, it will easily knock $1,400 off the mortgage every month.