They don't need to sue everyone, just enough to scare you. If your next door neighbor was sued by them tomorrow, I'll bet most of you would be wiping your harddrives by nightfall.
BTW, how many of these cases have gone to trial? Last I heard it was 0. RIAA is basically extorting money from these poor souls. Its cheaper to pay them the settlement than it is to hire a lawyer.
Personally, I'm waiting for them to sue someone who happens to actually own a copy of everything they have downloaded, on vinyl and 8track, and who has a friend who is a lawyer, or just has the money to blow on a lawyer, just to knock RIAA down a few notches.