“In October 1948, after the McDonald brothers realized that most of their profits came from selling hamburgers, they closed down their successful carhop drive-in to establish a streamlined system with a simple menu of just hamburgers,cheeseburgers, french fries, shakes, soft drinks, and apple pie.
The carhops were eliminated to make McDonald’s a self-serve operation. Mac and Dick McDonald had taken great care in setting up their kitchen like an assembly line to ensure maximum efficiency. The restaurant’s name was again changed, this time to simply “McDonald’s,” and reopened its doors on December 12, 1948.”
Wikipedia: “History of McDonald’s”