How come there's this echo reverb thing happening when you order at the drive thru?

Remember as a kid when you tied a string through a hole in two cans and stretched the string out tight to talk with each other. Same Idea with a drive thru! LOL

They use the radio for their intercom and your voice echos on the speaker thing. I had a shortwave radio and it would pick up the people talking at a drive-through fast food place a couple blocks away.

It echos so they dont have to get your order correctly

That's the subliminal message telling you to super-size.