We do that all the time, we just don't usually consider them to be robots. Car factories have all kinds of vehicles transporting car parts along tracks, ceiling

We do that all the time, we just don't usually consider them to be robots. Car factories have all kinds of vehicles transporting car parts along tracks, ceiling mounted rails, etc. Nobody bats an eye about that. If you squint hard enough elevators are a kind of robot on a designated pathway. But to be called a robot it has to have some human-like (or at least animal-like) quality. Which usually involves either imitating human arms (e.g. typical industrial robot arms), imitating human movement (e.g. Boston dynamics) or using infrastructure designed for humans. If you put a delivery robot on a purpose-built path it has none of those qualities and reverts back to being a normal machine.