Sidewalks and parks cost $0 for people to walk on, or let their dog shit in. Maybe $15/hour for the guys in blue jumpsuits to clean it up once a week?
NYC has the largest subway system in the world _by_far_ (and also one of the oldest). It's extremely expensive to operate.
Don't compare it to walking around on sidewalks or in parks.
It absolutely does not. It is the 12th in length and 10th by ridership: https://en.wikipedia.org/wiki/List_of_metro_systems
There are neighborhoods in Queens and Brooklyn where the closest subway station can be miles away, and there is an entire borough in NYC that has no general subway access at all, so not buying that one either.
An extra walker a day on a sidewalk costs the same marginal $0 or so an extra rider on the trains does. Both are expensive to run on large scale.
Not the case with the 4 train, or the L, or the 2-3. Those trains can't fail. So it's very expensive to make sure they don't. Nobody cares if Madison Square Park is blocked off for a day. (Maybe except the vagrants).
But the city will literally grind to a halt if the green or red lines aren't running.