Matt Damon and John Krasinski co-wrote, co-produced and star in Focus Features’ “Promised Land,” so naturally the small-town drama that presents them as opponents boasts a karaoke scene.Why are there so many karaoke scenes in movies? Damon and Krasinski couldn’t answer that question at the Dec. 4 preem at Lincoln Square Cinema, but did talk about why they put one in their pic. “I definitely wrote in that I was not going to sing karaoke,” says Damon, “but we needed a scene where John’s character appears and wins the town over. So we thought, ‘What better way than singing a great Bruce Springsteen song?’ “ Krasinski recalls, “The greatest appeal was in writing the stage directions: ‘This character is terrible at karaoke.’ The thrilling part for me was I didn’t have to be good after that. And we actually reached out to Bruce Springsteen for ‘Dancing in the Dark,’ which was phenomenal.” Significantly, there was no karaoke at the Loi afterparty.