Plastic back Alienware Area 51

Hi 

I received my laptop, it's an Alienware Area 51 16". I don't understand why the back of the laptop is made of plastic? I mean the part where the fans are installed. I think it should be glass. I looked at a lot of unboxings and reviews but I didn't see anywhere that it was a plastic black part...

Is there anyone who can explain and say more about this?

Thanks