Also, to your specific point - when people choose restaurants they often go where there's a crowd (the food must be good here), not to an empty restaurant. The situation is a little different because Starbucks is so pervasive, but don't discount the customer draw of a bustling store full of people.
Over the years just kind of casually observing the placement and layout of the stores, it seems that they are maximized to be easy to access by the morning rush crowds as well as the lunch and after work rushes. Also, a pronounced increase in drive-through locations.
I could imagine that the majority of a location's revenue comes from walk-in/out business during peak times, and the tables could stay full with little rotation all day without appreciable impact on revenue, while solidifying the chain as a go-to "third place", especially for the business class.
Any former employees here to chime in?