I'm sure the major factor is that bikes are designed to be fun to ride above all else. Aero is a factor, but a bike still has 1/4 the frontal area of a small car.

The bike I linked wasn't a Ninja, it was a 250cc "entry level" bike. I know a performance bike isn't going to get the best MPG, I didn't bother going there.

You talking about the new 49cc Ruckus? I found this statement in a review: "The Ruckus is abysmal for street riding, unfit for the majority of public roadways." Seriously?