An app is not a document in Mac OS (and other operating systems). Therefore it is logical that closing the document will not quit the application. Treating all applications as document is a flawed model which often breaks down as is made obvious when opening multiple documents in a MDI system.

It's even traceable all the way back to the user interface research at Xerox which Apple took and went with because it worked so well. Researchers such as Donald Norman also concluded that MDI interface were much worse at common actions such as drag-and-drop and standardised interface controls.