They can’t use metal wheels because of the aluminum rail used to support this movement. Aluminum is highly susceptible to a process called “galling” where dirt and other contaminants create small metal shavings that then get ground into the metal surfaces. With force and time this causes the surfaces to get rough which then increases friction and heat. Depending on the metal pairs, the galling process can be very fast, if you have similar or identical metals, or it can be slower, but it will almost certainly happen. The final result is that your machine gets noisy and then the wheels stick to the frame and your machine is now frozen up permanently.
The other drawback of metal wheels on metal rails is that they require lubrication and they are noisy. A lubricated wheel would need to be protected from the debris of operation, which will drive up cost. The lubricant would need to be replaced pretty much every time you use the machine since it would, by design, soak up all the contaminants that come into contact with it. So now instead of just cleaning the lenses and wiping down the rails, you have to wipe dirty grease off the rails and wheels and anywhere else it may have gotten.
For an exposed roller design at this price point, a polymer wheel is really about the best solution.