They absolutely could have, and have been entirely within the rules of the DMCA. My best guess is that the ContentID system is either (a) a bribe to get media

They absolutely could have, and have been entirely within the rules of the DMCA. My best guess is that the ContentID system is either (a) a bribe to get media companies to partner with youtube's paid streaming, or (b) a bribe to convince media companies that it isn't worth lobbying yet again for stronger copyright monopolies.