I'll tell you why... Apple tells them that it's premium and it's the best aluminum. Well honestly how much more can aluminum cost over polycarbonate? It's just as Apple says the polycarbonate on the 5c is the best plastic. Well it's plastic just as aluminum is aluminum.
The iPhone 5c is a well built plastic phone. When you hold it compared to a galaxy S4 it feels much sturdier. It's not a pos device by no means.
I'm by no means saying the 5s isn't a good phone because it is, but to say it's much more premium because it's "metal" is completely absurd. I have many soda cans in my trash can right now and those are not "premium."
When you look at the important stuff in our everyday life ... How many has some company made out of aluminum? How much of it is made out of plastic.
Aluminum is cheap it doesn't make something premium and IMHO the 5/5s is a major major design fail.
I don't understand how a device can't be considered premium because its made of aluminum. Besides many other Apple products that are made of aluminum, aircraft, engine blocks and body's, load bearing architectural components and many other things are made with aluminum. It's not the materials but the execution that make the difference between average and premium. Just look at a stamped steel chefs knife at a dollar store and a Wustof chefs knife. Same basic material, cosmically different user experiences.