It's not the first time I hear it, but I've found it in this scene. I understand that, as in 知る or 始まる, a started action whose consequence remains is expressed in continuous form. However, I thought that the negative form was constructed in present or past, as 知らない or 始まらなかった, but not in continuous. I find examples as the linked video using instead the negation of the continuous （見ていない）. In this case I would've used 「見なかった」.
Am I missing something or is it a slang transformation? In that case, which form should be used? Thanks a lot!
EDIT: corrected misspelling/bad grammar. Tremendous lapsus :(