There have been tons of articles recently about how rising fast food prices are causing people to consider it a "luxury", and a huge part of those rising prices are rising wage costs. Where I live all the McDonald's now have kiosks inside for orders and fewer cashiers. I absolutely think shaving labor costs is something all fast food joints are interested in.
I do agree with the other comment that drive-through workers multitask (scooping fries, filling drinks, etc.), so I'm not sure how much savings you'll get just from order automation.