Its not even that. Imagine another industry where this would work? it just doesn't.
Video games are announced, even if its just a title and a brief description, 2-3 years in advance, and then drip fed images, trailers and developer interviews until it comes out. And it doesn't harm sales does it?
Same for the film industry. Previews, sneak trailers, real trailers and then release.
Hell some companies deliberatly "pre-leak" images. it gets the public more interested