Josh Ozersky calls it a "sentimental tradition":
The thing to remember about Chinese food is that, besides being cheap, it is eminently suited to take out; at least three-quarters of the Chinese food I ate growing up was at home. And Jews love eating at home. We are intensely familial, home-loving and nuclear; and given that our own food is both bad and laborious (endlessly braised brisket, spattering latkes), Chinese food – varied, fatty and festive – is a better alternative in part because it's always at hand. It's a cheap lift; you can think of it as Jewish Prozac.
(Photo via Punny Signs)