Because sucking down one of these while in the car is highly inadvisable. Although they do offer three sizes just like a regular Starbucks. Instead of Tall, Grande and Venti, you can have your coffee either Eldritch, Chthonic, or the mack daddy Non-Euclidean. They also have a very Short version called a Gibbous, but you have to ask for that special now–it’s no longer listed on the menu.
Mug found via Dagon Industries.