Because if the devs that do these things think consumers can't be trusted and most be monitored 24/7 or they'll lend their game to a friend and that friend would be a potential sale lost. Even if pirates will crack the DRM open in like a week, and the only thing that these stuff causes is lost sales, like the SimCity debacle, companies like EA or Microsoft believe they really stop piracy, instead of learning that piracy is good for the industry because it helps potential costumers pick games, so they'll keep forcing it down our throats.
Sometimes knowing Nintendo doesn't get how the Internet works is actually a relief.