While what you say is true, getting a box for leftovers is an entirely different animal than getting a to-to order. To-go orders totally bog a kitchen down, while you boxing up your own leftovers after having your server bring a box with your check has no effect on a slammed kitchen.
So yeah...while SOME do have to-go boxes, there are very good reasons they don't offer meals to-go. Rusty listed several above.