The problem with this is that they don't know before takeoff how long you're going to have to circle, hold, or be vectored around once you hit NYC airspace 2 hours later. The two choices are to plan for the potential delay, and end up with extra fuel on board if the delay never materializes, or to not plan for the extra delay, then force a diversion if such a delay does occur.

The way the system is currently set up, if they knew for certain that you'd have to hold/circle/be vectored excessively upon arrival in NY, then you would have your takeoff delayed to avoid airborne delays... This is why we have wheel-up times, more formally known as EDCTs (Expect Departure Clearance Times). The powers that be realized long ago that it is safer and more efficient to delay a plane on the ground than in the air, but the predictions aren't always perfect.