You had to reach back into history for that one, huh?
I have seen more heroics in NYC, before, during and after 9/11, than you will see in a lifetime outside of a war zone.
I may have had to reach back, but I sure did have a perfect counterexample, didn't I?
As to the 'heroics' you've seen in NYC, I'm sure you have seen plenty. With more people there, odds are good you would see more 'heroics.' Heck, I'll even bet you do see more than I do in my lifetime.
But that doesn't mean that the town is full of heroes.
And btw, war zones don't make heroes. Escapable yet willing self-sacrifice makes heroes. Someone who shoots an enemy soldier with a gun pointing back at them is not necessarily a hero. Even someone who takes out a whole enemy regiment isn't necessarily a hero. But someone who willingly exposes himself to mortal danger to save others, when he could easily escape that danger himself--that's a hero.