Yes, "why" is a great question to ask, because ISTM Alphabet benefits from being able to blame an essentially unappealable process while it curates content uploaded to its platform by creators. I'm a rancher, and like Alphabet I prefer not to get in extended arguments with my livestock.
If blame for the excesses of curation couldn't be offloaded to third parties, creators could form a class for enterprising attorneys.
Sure, it's also good to have a ready defense against litigious record companies, but large firms don't have to have just one reason for doing things in particular ways. It is easy to imagine an alternative to the "ContentID" system that would be less capricious and would still make record companies happy.