Because the author ends up duplicating features of the zip format. Think of all the stuff that goes along with a ppt slide. Fonts, images, videos, sounds. You w

Because the author ends up duplicating features of the zip format. Think of all the stuff that goes along with a ppt slide. Fonts, images, videos, sounds. You want to embed the original files anyway, because storing the content in your own custom format is pointless extra work. think how complicated a video file format is. It's a good fit for a file archive.