What service would you be tipping for at a drive-through? You literally were made to drive a several thousand pound deathtrap up to a specially made window so you could awkwardly exchange goods through an open car window with exhaust fumes bellowing about. No one is going the extra mile for you. By comparison, at a drive-in, someone went out of their way to come up to your car, ask you what you want, go back inside, have it made, and bring it straight out to you. How thoughtful and pleasant, tip city!