The reason they haven't is that you're wrong. The DMCA doesn't stop working when you get big. It stops working when you take steps to ensure that infringing content is featured on your site.
For a bulletin board full of nerds, we sure are happy to insult each other's intelligence about what MegaUpload was. What kind of an idiot do you have to be to not understand what made MegaUpload popular? Are you really willing to write a comment in public under your real name saying that MegaUpload was a good piece of software? That sentiment doesn't make you feel embarrassed as an engineer? It was a crappy piracy site; it was popular solely because it was an effective way to upload and download pirated movies and TV shows. If it had been a Dropbox competitor, we'd all be making fun of it, instead of pretending like its founders are technology martyrs.
The pirate bay case is an example of a website, which though safe harbor would protect them in regard to user-uploaded content. They where wrong in that regard.