Because a "drive-in", like Sonic, you have to stop, and order your food. The "drive-thru" means you don't have to park your car. Did you ask the question about parking in a driveway, and driving on a parkway a few days ago? I can't remember.
I don't know, but a friend and I have plans to stop that. We call it "The Food-mobile". We drive around cooking things like tacos and hotdogs. We drive up next to people and they tell us what they want, and we throw it out our window into theirs.