This can’t be true. Just look at the homelessness situation across the US. No shelter, no income, no sanitation. That is certainly worse quality of life than a king.
You must starting your definition of poor quite a ways up the income scale. Maybe you could make make an argument for solidly middle class and above?
Even then, there are even a lot of benefits to being a king which probably outweigh having basic modern amenities like running water and a nice bed. Who needs plumbing when you have a servant to bring you whatever you want?
Beyond that, quality of life is also significantly emotional. There’s a lot of stress to life unless you’re rich, and you also don’t have very much control over things around you. A king has a significant amount of control by definition, and that probably contributes a lot to their quality of life.