Why? It's very common in English:
I don't want those ones.
In this case the number has been repurposed as a placeholder noun, and the noun is what's pluralized, but that's also what happens when the number is repurposed as a determiner and the determiner gets pluralized.
I think it probably just boils down to whether the word is functionally necessary to understand the meaning of the sentence and how the language evolved...
For example, in French you don't usually pronounce the "s" at the end of a plural noun and lots of singular nouns end in "s", so the articles become necessary for determining if you mean plural or singular. In English you pronounce the "s" clearly and very few singular words end in "s", so you just don't need the article to distinguish anything, it becomes functionally redundant.
"*a scissors/pants" is correctly "a pair of scissors/pants" and you still use the definite "the scissors/pants"
So "get me scissors" is a bit odd but works only because the singular item happens to have a plural name.