I was told many years ago by a native Japanese speaker that saying 知っていません instead of 知りません was a huge mistake. I would like to know the reason of this fact. Is it because the meaning of the verb 知る expresses by itself the progressive aspect ?
The reason seems to be unknown.
According to "A Dictionary of Japanese Grammar", page 407:
When answering in the negative to the question 知っていますか, the negative nostative form 知らない/知りません is used instead of 知っていない/知っていません.
But the book does not explain why.
知っていない should be the grammaticaly correct form.
Maybe somewhere, someone, somehow, started to use 知らない instead of 知っていない and that became popular among the general public, to a point that now nobody uses 知っていない.
There are some people trying to offer explanations, but they look more like speculations to me:
『新日本文法研究』第７章「知ラナイ」と「知ッテイナイ」p.109－p.116大修館書店1983年 著者: 久野暲