The trouble with using hashes is that they have no context. Suppose I own the copyright on some file and issue a DMCA takedown against somebody else sharing it with the world. In that context the takedown is totally legitimate. But now the hash is on the list, so what happens next time when the file is being shared in some totally different context? What happens when the copyright holder wants to share the file? What happens when some other third party wants to share the file with a small number of people in a way which is unambiguously fair use?

You end up over-blocking, unless you include some option to bypass the block, in which case why even bother? It's not like you could inform the copyright holder when someone shares the file without invading users' privacy and opening up the system to abuse against political speech.

The whole thing is a mess because the fundamental problem is that the cost of most infringement on the internet is less than the cost of evaluating whether that use is infringing, so you end up with a high error rate no matter what you do. It breaks the founding assumption that copyright is not in tension with the First Amendment and makes you choose which you want to sacrifice for the other.