But why do we limit the eruv to our city? Why not country or world? Is the idea that other Jews in our city can be expected to share the food we've prepared for shabbas (or use it in their own meals)?
The Talmud (Beitza 16b) writes:
The Mishna Berura (OC 527 sk 29) notes that the need is for the person to be able to come take from the Eruv food. This makes sense because setting aside food that is unreachable can hardly be considered beginning to prepare a Shabbat meal.