Even if you're only going out to get groceries, NYC is a petri dish. A lot of walkups, and I imagine old people need to use the railings on staircases. The interiors of these old buildings have poor ventilation. And New Yorkers are so impatient, I can't see them riding an elevator one at a time, especially since residence buildings with elevators are usually huge with tons of people.Cuomo noted yesterday that 2/3 of hospitalized cases in New York were people who were sheltering in place at home. He had expected most to be essential workers. So case in point.
I wonder how many of those people really understood the principles involved with isolation however.
I can see how easily it would be spread in NYC even if everyone followed the rules. Mix in the subway and it's a lost cause. In other more spread out areas, sheltering in place probably works a lot better than nothing.