I thought that myself as I posted it, OP.
I personally always think of cocktail dresses as knee length.
What I liked in this dress was the shape of the dress, a fitted bodice then a full skirt. I remember cocktail dresses looking like that when I was growing up.
Something else about this dress is the lustre to the fabric.
One of the things which says 'cocktail dress' to me is a luxe aspect to the material, with a lustre or even a sheen.
My recollection from years ago was that cocktail dresses fitted into early evening gatherings, before the theatre, that kind of thing. The lustre in the fabric is picked up by the artificial lighting.
It's the half way house between day wear and full on evening wear.
With this dress, the closest to the 'look' would come if a seamstress took some inches from the skirt to take it to the knee.
I appreciate that my idea of a 'cocktail dress' might be considered a bit old-fashioned, however.