I still agree that congestion control, disincentives for car use, wider sidewalks, all should be done. Some of us wish NYC could declare independence so we could choose our own policy, but that's a different story...
One of the reasons for this is that street parking is usually subsidized well below the market rates and so drivers will spend a significant amount of time looking for a cheaper parking space rather than simply parking in a garage, and they're using road capacity until they either find a space (and then block the lane while they slowly attempt to parallel park) or give up and pay to park off-street:
“In 2006, surveyors interviewed drivers stopped at a traffic signal in the SoHo district of Manhattan, and 28 percent reported they were cruising for curb parking. A similar study in Brooklyn found that 45 percent of drivers were cruising. The same results might be found on many other streets in New York because off-street parking is generally far more expensive than on-street parking. In midtown Manhattan, for example, the price for the first hour of off-street parking is often about $20, while curb parking is only $1. Parking for an hour at the curb saves $19, but drivers first have to cruise to find a space on the street.”