Except that it can. Youtube lets producers (channels) post polls, let's consumers (viewers) post comments, and if channels want to, they can do something interactive- over a video series & etc.
Youtube has no interactivity within videos. There used to be a little bit of it with annotations, but those were killed off ages ago.
Compare that to Flash video series like Homestar Runner, every episode had little Easter eggs where you could click on background characters to unlock a bonus scene about them. It really helped you feel engaged with the medium in a way unmatched by anything but full-length video games.
