Geographical based sharding seems to break down once people travel though. Just a single visit to a hub airport might have gotten you in contact with people form all around the world (I assume that the objective of this initiative is to try and get us at least part way back to normal). Even if you don't travel, but other people are, you will be in contact with people who are registered as infected in a different region.

Also I don't think NYC is at all the worst case in the world, there are a lot of megacities that dwarf it in size...