In a really big and busy city it's emotionally exhausting and not reasonable to have an interaction with everyone near you. The only way a lot of people can tolerate being packed into busy public transit systems on a daily basis is to intentionally ignore each other to a certain degree.

It's essentially the same unspoken etiquette rule as what you're socially expected to do if riding a crowded elevator.

Go commute by NYC subway 10 times a week, M-F especially during peak tourist season and you'll understand.

I intentionally behave completely different if I'm in a small town of 3000 people or walking down the street, shopping, riding transit in a large city.