Why do we generally ignore the boundary of a solid when we describe its structure? Because the vast majority of the units that repeat throughout the solid are inside it, not on the edge.

Imagine the entire North American continent with no cities, no buildings, no roads and planted as one vast tree farm. If you were in the middle of the continent, the edges would have no impact on how you viewed the structure of the world around you. The same is true of an atom inside a crystal. Most atoms are so far from the edges that the edges have no effect.