God's "point of contact" with us as unsaved humans (sons of man) is that giant "hole" in our heart. Satan's point of contact with us is our ego.
God interacts with "nothing." (It's that "hole" that hungers only for God and is filled and satisfied by nothing else.)
Satan interacts with "everything." (After all, isn't that what our ego drive amounts to? Aren't we basically living only for ourselves?)
On the surface it seems like a losing proposition. But God is clearly in his element. Going back to Genesis 1, we see him delighting to create something beautiful out of "nothing."
Nevertheless, Satan needs to start somewhere (since he can't create). He has a lot of "raw material" to work with in the form and shape of a human "ego." The bigger the ego, the more Satan can play with.

Thus the battle lines are drawn.