They are thinking, where you are not. Is the cost savings per phone minimal? Perhaps, and it might not make that big of a difference if you made only one phone. However when you sell as many units as Apple sells, those minute savings add up in rapid fashion.
By the way I can assure you the cost difference between molded plastic and tempered glass/machined aluminum is substantial.
Just for comparison ...
Scrap Plastic brings from .02 cents per pound up to .26 cents per pound.
http://www.plasticsmarkets.org/plastics/pricing.html
Scrap aluminum (cans) brings .72 cents per pound
http://www.metalprices.com/p/ScrapAluminumFreeChart