Because the sex is still the point of the interaction with the media and hence creates an idea where the idea that interaction may solely be sexual and that is what causes objectification. And the difference with the idea of imagination is two differences. One being that it is nearly impossible to regulate and the other one being that the point of imagination is not sexual meaning that it serves other purposes. (And I am sorry if I come off as rude or something, I am trying not be at all but it can be hard to be clear of emotion of phrasing on the internet