A small, fuel-efficient, "sensible" little commuter car would probably take car of the daily transportation needs of 90% of us, but it's about choice. Americans like to have a wide variety of choices when they go car shopping.
Let's face it, nobody needs a gigantosaurus 4wd SUV to drive themselves to work every day. But that's what some people want. Does it make sense, logically? No, of course not. But as long as gigantosaurus SUVs are legal to own and operate, and the owner can afford the gas, hey, that's their choice and their business.
Same with full-size coupes. Are they for everyone? No, but there are those who prefer them. Does a full-size coupe make sense to the type of person that drives a Prius? Probably not... but then again, the full-size coupe guy probably doesn't see the point of the Prius. Different strokes for different folks.