DRM doesn't stop you from playing pirated content, the goal is to make the content only decryptable using approved hardware/software, to limit people's ability to copy and paste content Willy nilly and share with their friends in Napster fashion. The determined people still stealing content never will be stopped, but it's inconvenient enough that average people aren't going to duplicate DRM-laden stuff themselves.

Importantly, I think, when there's a higher barrier to content theft, the remaining sources of pirated content are fewer and easier to track.