> Soldiers move in 2D, missiles in 3D.

Infantry troops still operate in 3D as they can/will direct mortars, air assets, etc.

In general, what you laid out is a bit of an apples and oranges comparison. An infantry platoon applies fire and maneuver using all of its elements to "close with and destroy" a smaller element.

You emplace your weapons squad which has machine guns and other large weapons (e.g., mortars) that will keep your enemy's heads down. This allows your maneuver elements (i.e., assault squads) to maneuver to the enemy force.

In the case of missile defense, you have a limited set of complex and expensive bullets, and your enemy has cheap bullets. If you want to defend yourself, you're attempting to hit your opposition's bullets with your own bullets.