You actually answered your own question. Its NYC. A friend of mine told me she was driving in NYC (not in Brooklyn) and couldn’t stop laughing when people just crossed when they felt like it, regardless of there being a car speeding down the block. Everyone makes their own set of rules.
I do agree about using a blinker as often as possible, but I think its one of those things that are associated with new drivers and new drivers only. Its a shame because, as you say, it can literally save lives.