"Why is it always 'polymer' and not what it really is , plastic?
Why is it always "lightweight alloy" and not aluminum?"

Plastic and polymer are not actually synonymous. In fact the term "Polymer" is a little more precise than plastic, which simply means "moldable." But even at that, there are hundreds of different chemically-diffferent polymers (nylon, teflon, polyethylene, styrene, polycarbonate, and on and on). Lightweight alloy could encompass numerous metal alloys based on aluminum or magnesium. A better generic descriptive term would be aluminum alloy (or possibly magnesium alloy), but then there are a great many different aluminum alloys just as there are a great many polymers.