In-N-Out is a low-margin, high-volume business, kind of like how Walmart is.

They literally can't cut staff because they only make money due to the incredibly high number of meals they serve per hour. So they have to serve as efficiently as possible, which requires that fully-staffed kitchen and counter, or there wouldn't be enough time in the mealtime hours to do that volume otherwise. It's just as impossible for them to cut any staff as an airline could remove pilots from the cockpit, in that the whole operation would cease to function.

McDonald's and the rest are never going to be In-N-Out though because it would take years of operating at steep losses in the hopes people would notice how fast and tasty they'd become. I can't think of a mature company that's ever pulled off going from "boring mainstream place" to "cult favorite" like that.