I think "the only acceptable" in the GP should be read as "the only thing we should accept", as the rest of the comment is really pointing out how Youtube's system is gamed by the large media companies.
There are penalties for false DMCA claims, so allowing companies to bypass a real DMCA claim lets them shirk a lot of work (and responsibility) for making sure those claims are correct, lest they incur legal or financial penalties.
Because YT chose to insert themselves in front of the DMCA.
When someone copyright claims your video, they're not actually sending a DMCA, they're doing a YT-specific process that just happens to look sorta like the DMCA notice/counternotice process without involving the legal system.
There are penalties for false DMCA claims, so allowing companies to bypass a real DMCA claim lets them shirk a lot of work (and responsibility) for making sure those claims are correct, lest they incur legal or financial penalties.