It’s probably largely for historical reasons: the first real piece of “rational fiction” was Harry Potter and the Methods of Rationality by Eliezer Yudkowsky, and many other authors followed in that general vein.
Also, it can be fun to take an existing work with a world that wasn’t very thoroughly examined and “rationalize” it by explaining plot holes and letting characters exploit the rules.
It’s probably largely for historical reasons: the first real piece of “rational fiction” was Harry Potter and the Methods of Rationality by Eliezer Yudkowsky, and many other authors followed in that general vein.
Also, it can be fun to take an existing work with a world that wasn’t very thoroughly examined and “rationalize” it by explaining plot holes and letting characters exploit the rules.