I sort of imagine that these puzzles have an outlier effect: it removes the top and bottom users from a selection pool. Maybe that's okay, personally I was a bit disappointed at the missed opportunity.
One way they could improve this is by giving the puzzles expiration dates, removing the fear of completing one far ahead of the interview process, and having it expire. You'll still miss out on those who haven't heard of ITA until the near-graduation career fairs, but it will at least improve the situation.