There are still many games without 20 ratings.
Why should you rate those games? Their creators don't rate back.
1. First of all, if more games get a rating, your ranking will have more meaning. (You came in 63rd out of 1000 sounds better than 63rd out of 500, no?) 2. Giving feedback to those who need it. They will learn, and I think thats what Ludum Dare is really about. Comes with free Karma. 3. There are some hidden gems. (But they are running out, quickly find them before someone else does!) 4. Admittedly I got most reviews before rating any games, I got a bunch of people rating back, even from the ones who hardly rated other games.

Find games to rescue here:
http://feedback.ld.intricati.com/?event=32802&sorting=received
(At the time of writing there are 14 games without any feedback - not even a "download link doesn't work")

There are other groups of people that rarely get any ratings. For example this list of 16 VR games.
http://feedback.ld.intricati.com/?event=32802&platforms=vrgames&sorting=received
There's also a hand full of board games
http://feedback.ld.intricati.com/?event=32802&platforms=board&sorting=received