This is ridiculous. When I throw parties I tell a couple of my friends and tell them to tell others and people just show up. Americans are living in some sort of parallel dimension.
Reminds me of an acquaintance who told me he was an introvert; he said after 20 hours of being around people he'd need a couple hours to recharge.
Sometimes you also need to know who will be there because if half the group flakes out, the logistics fall apart. Not every party is a house party.