Why do people use the drive-thru at a drive-in restaurant when they have absolutely no idea what they are going to order?

Why not drive in, ponder the board, decide what you want, then push the “order” button? Isn’t that common sense?

I was at Sonic today and I saw a couple occupy the drive-thru for over 10 minutes. I know because I pulled up behind them (they were the only car in line) at 3:55 and half price drinks end at 4:00. Having had this experience before, I pulled around them into a stall and ordered my drink at 3:58.

This is not an uncommon experience at my local Sonic. Recently I sat behind a woman with an SUV and 3 kids for several minutes before I pulled into a stall. My drink was ordered, and delivered and I was pulling out at the same time she was pulling up to the pick-up window.

Stupidity? Selfishness? Obliviousness?
What causes such behavior? If there are more than 2 cars waiting to order at drive-thru, I just pull into a stall and order.