Firstly, YouTube don't tend to support downloading videos as raw video files. So it wouldn't really make sense for YouTube to provide a signature when you're not really supposed to be able to access a video file in the first place.

This also wouldn't really work because a YouTube video isn't a single file. Videos are available in different qualities and formats. The video component tends to be separated from the audio component. If you want to combine them into a single file, you have to mux them. However, this process is not deterministic, so the resulting hash would differ depending on a multitude of factors.

> No need for a signature, just compare the hash.

Then Youtube would have to publish the hashes for each and every file that they encode the source material to. Practically impossible and especially unverifiable if YouTube ever disappears (or the source gets deleted).

A digital signature of the hash, embedded in the metadata and computed similarly to Reproducible Builds... doesn't take much space and especially only requires knowledge of the public key for verification.