I believe Lockhart --- Berger didn't stuff documents in his socks --- it would be too hard with most socks to put any kind of documents into them and think they wouldn't fall out and you'd be caught. He also didn't stuff them down his trousers because there would be nothing stopping the documents from sliding down and falling out the pant-leg opening. He stuffed them down his underpants so he could walk out knowing they'd stay in place.
Berger's pilfering gives new meaning to classified briefs.