> I just wonder why they choose not to enforce this aspect of it when it can be a significant population (in a statistical sense) of the car traffic as well as revenue.

It's not. 85% of Manhattan households don't own a car at all. The number is even higher inside the Congestion and Relief Zone. Almost all car traffic within the zone is from people who do not live within the zone.

I wager they did a study to weigh the costs and came up with the inevitable answer that chasing down the tenth of a percent (a wild guess but I think it's at worst an order of magnitude off from the actual number) of cars that exist only within the congestion zone and never leave would cost more than you'd gain both in fees and second order effects. Especially the population of cars most likely to never or rarely leave are being charged on a per ride basis, cabs and rideshare vehicles like Uber etc.

It's like all the efforts to drug test people on welfare, they cost vastly more than they save/recover.