Because, for all of its sins, the DMCA provides one very crucial piece of legislation, without which the web as we know it would crumble: Safe Harbor.

Without the Safe Harbor, people wouldn't be getting takedown notices or content ID - sites like YouTube, Reddit, Hacker News, comment boards, blogs... all would have been sued into oblivion years ago, as they would have been held liable for the content uploaded by their users.

Fixing the DMCA, then, requires a precise modification of the law, not just a straight up repeal - and getting that done without loosing ground in other areas would be a tough job for any lobby or politician.