If you delete your video from youtube, they have the right to store it on their servers until they want to get rid of it, and you're allowing people to use your content, i.e reactions videos, which are oddly popular these days.
Wait, what? How are other people allowed to use the content? I mean, I'm sure it happens, regardless of being allowed, but it seems like a content owner would be able to issue a DMCA takedown to stop it.