Rules of thumb are how the ancient Greeks built cathedrals that still stand today. They were developed not through theory, but instead through passing down the techniques used that Empirical observations proved to work. They tried things. The things that worked, they kept on using.
Rules of Thumb are what gets used when the situation is not fully understood.
They may not be perfect (in fact, they almost certainly aren’t), but they are “good enough” for Satisficing the situation. If they stop working, they stop being passed down and used. The Things We Make includes rules of thumb in its definition of ”The Engineering Method“.