Because they certainly do not gather around the pond edges, as they do in warmer weather.
I know this because I just took a winter walk in the park and gravitated toward one of the ponds, where a few months ago, there were turtles galore. Not today.
So I came home wondering.
And here’s the answer, from Central Park’s Ranger Rob (a human being, not a turtle):
Turtles are an inevitable summer sight, resting along the shores of Central Park’s water bodies. But during the winter, it’s as though they just disappear. But where to? In what Ranger Rob calls an “incredible adaptation to avoid the cold winter,” Central Park’s turtles bury themselves under mud at the bottom of frozen water bodies. There, they hibernate, conserving energy until the water thaws and it’s time to head up to warmer temperatures.