[Stratoscope]

> nor can you ask for "a scissors"

But you can ask for "the scissors".

Why does that sound OK but the other does not?

(Not debating, genuinely curious.)

[Skeime]

Because scissors is plural, which does not have an indefinite article. You can ask for scissors, like you can ask for documents. (And you cannot ask for a documents, only for a document.)

[tuatoru]

A pair of scissors, a pair of trousers. I shortened the trousers with scissors.