It’s just that no-one on the show thought of it.
My feeling is that we should separate the creation and the result, and keep them clear in our heads.
Fiction will never be a complete picture, it’s a sketch. We’re looking at the bits that the people who made the show (or film or comic or novel etc.) thought were important at the time.
Those people are real, they have lives, schedules, responsibilities (in and out of work), strengths, weaknesses, blindspots, personal problems and successes.
And sometimes they fuck up.
‘Firefly’ was loaded with asian text, names and design but no-one thought to cast an asian person on the crew of the ship.
That was, in the end, a fuck up. The best way to reduce the number of fuck ups is to admit to them when they happens and keep them in mind when going forwards.