I get your confusion. I was actually reacting to this part of your comment specifically. I am for what you are saying. I just wanted to mention that modern buildings aren't always designed like that. In fact, where I live, houses seem to be increasingly designed to only keep the heat in, not let it out — as rightly pointed out above. That's a problem. We basically can't stay comfortable in summer sometimes. It's not pleasant sleeping on the upper floor during a heatwave and there's little we can do other than buy an AC (which we won't).
As a sidenote: our company just finished a new building which is designed in a similar way, but has active cooling via the heat pump/floor heating system. I hope that will work better than what we have in our private home.