Mainly because most formats require a pretty complete file to work, and the way P2P networks work is you download whichever piece that's available. The way videos work is you have a keyframe, which contains complete info, and then subsequent frames only contain the difference, until a scene change, or a new keyframe at fixed intervals. If you get a piece of the file, it may or may not have a keyframe, and thus it may or may not be viewable. You kinda get my meaning here?