Because it was never the point, never really written into the show from the word go and only really was ever considered when various showrunners realised that fans were expecting it.
Trek is about a message, about ideas, the setting is just that, a setting which allows those ideas to be expressed. It was never supposed to be a Tolkien esque epic world to be explored, which is exactly why it falls apart so readily under analysis.
It's ironic that for some reason it is singularly prone to attracting fans who are looking for exactly the opposite, who see the ideals of inclusivity, tolerance and diversity as less important than the deck counts on the ship or the appearance of Klingons.
Just to demonstrate how ridiculous the irony of this can be, we've literally seen people arguing against representation of disabled people on the show, something which is exactly in fitting with the purpose and ethos of trek, why it has such an enduring cultural impact, because they don't feel it fits in with "canon" about the medical technology on the show.
It's hard to imagine any more clear an example of missing the woods for the trees.