They do have humans, but they make mistakes and have bias. And if we need to cover the amount of content Youtube gets per second we'd need a country of people.
This is about disputed copyright claims, not all videos. Besides, most of these copyright holders probably file hundreds of automated claims (from examples in the thread: chiptune samples, royalty free drum loops, presidential speeches, CC licensed music). If it's obvious that it isn't copyrighted, YouTube can remove the fingerprint from the Content ID system and force the supposed copyright holder to issue a DMCA notice, which is much easier to dispute and works in favour of the uploader.