Why aren't there leaderboards for individual levels? I think it would be a fun thing to have!

There used to be, quite some time ago. From what I recall, it created a ridiculous backlog on the verification process and was removed. They used IGT, which meant a LOT of ties for first place in most of them... although it did allow for some unique and interesting IGT-friendly strategies you would never see in RTA in a couple of levels. There is 8-4 IL available on the category extensions (RTA timing), but I suspect the ambitious nature of trying to maintain the other 51 ILs remains too tedious.