It had to be pointed out that Superman was already so old, and had never gone on a trip to outer space from Earth.
It could only be said that human habit was just that incomprehensible.
That was because ruling out certain predetermined standards before thinking about a situation was a more practical and efficient choice so that people didn't have to refresh their understanding once again.
It was like when you wouldn't immediately think of a girl with long, wavy hair as cute (except on special occasions).
By the same logic, Luke had never considered that Superman might leave since the odds of that happening were too low, while the chance that Superman would run into an enemy that only he could deal with was even lower.
But even if the odds were one in a billion, it could still happen.
In any case, Luke never had Superman as a teammate back home, and Luke was the one who took the lead in taking down a bunch of supervillains.
