This has a number of problems. The most egregious is if I'm an attacker and I have the file you request I can hash the appropriate portion you'd use to verify

This has a number of problems. The most egregious is if I'm an attacker and I have the file you request I can hash the appropriate portion you'd use to verify it but fill the rest with junk or exploits. You'd receive the file, it would emit the correct hash, yet be not what you were expecting. For video especially what you receive isn't necessarily predictable by the client. With HLS or MPEG DASH streaming the video you receive could be one of a number of different encoding e.g. lower or higher bitrates to deal with changing network conditions. The actual m3u8/mpd file you might receive could change arbitrarily as the video provider adds or drops different encodings. The hash of such a file today isn't guaranteed to match the hash tomorrow for entirely banal non-malicious reasons. Fun fact: the UUHash algorithm used by the FastTrack network (Kazaa, Morpheus, etc) only hashed the first bit of a file. Hashing a large file took forever on hardware of the day. Even hashing small files was non-trivial. The RIAA through various fronts would insert spoofed files where the first portion of the file was legitimate but the content of the file would be junk or annoying sounds. The files would be named like any other MP3 someone was searching for and even have seemingly good IDv3 tags.