Why doesn't YouTube allow you to replace a video?

Sounds scary. If that were an option, I foresee increased rick-rolling, guerrilla marketing opportunities, and shocking of people. (Dialog of an evil person: "Looks like my video is getting enough attention, time to take advantage of my audience! Haha eat Rick Astley/our new Nike product!")

Also, aren't the comments, shared links, and codes embedded by people the result of the original video, not anything new or modified? It might be dishonest to keep comments pertaining to different content. Youtube is also a site used more for their embed codes than other sites. If you replaced your video of cute puppies playing with a chew toy with one of vicious polar bears biting the limbs of their captors, it would upset a lot of people who embedded that video or emailed a link to their grandparents.