I don't think it's that, I think it's because they last longer on the shelf (light roasts go stale faster) and because dark roasts just taste burnt, meaning you can swap out one bean for another and not notice a difference. So if a cheaper beans becomes available, that will get switched in.
Dark roasts are not more shelf stable than lighter roasts though. The oils in dark roasts become rancid substantially more quickly because a darker roast has expanded more, and therefore has more surface area to come in contact with air. Starbucks and other big companies say their coffee is shelf stable for about a year, but the reality is coffee peaks 24-72 hours after roasting. Burnt notes grossly outweigh staleness though. That with the benefits of consistency is great for bigger brands exactly for the other reasons you mention.
To add a bit more perspective, green unroasted coffee beans are considered fresh or in season for about 9 months. The more you get away from this in a roast profile the shorter the shelf life.