Most of these groups are simply “A” and “Everything not A”. Either a number is odd or it is even. Either a place is lit up or it is dark.

That being said there are also some cases in there where there is more than just two categories (like male-female or plant-animal) but we, for the most part, only think about the most important / biggest ones.

All of this probably comes down to the fact that in order to make sense of the world or brain constantly tries to put things into categories to quickly assess what something is or isn’t. And it makes sense that the easiest way to categorize things is by just going “Is this A or B”?