Guest bedroom is underutilized space unless you are a hotel or full-time Airbnb. How often people are staying there? A few months a year in total, if children spend the whole summer with grandparents and visit on rare holidays?

Is that a problem? It's a guest bedroom for every two houses. Something approaching the vast majority of all economic output goes toward things which are extraneous when measured against the historical median standard of living. But people like nice things. And a guest bedroom is a nice thing that some people like.

That feels... weird. Would you say a car is underutilized if it's not being driven 24 hours a day?