Not necessarily by design. If they're ordering fleets of bikes, they want them cheap. Steel bikes are the cheapest around, can take a lot of abuse, and also happen to be the heaviest.
Steel bikes don't have to be heavy but cheap bikes usually are. A good steel bike might weigh 19-20lb (compare with a medium- to high-end carbon fiber bike that is 15-17lb) while the frame is only 3.5-4 lb of that total. The rest of the components make up a lot of the weight and to save weight there you have to spend more.