Many expensive headphones use plastic because of weight. Even the Sony MDR-Z1R have plastic. One of the only headphones I know that don't was the Sony MDR-R10, but from my understanding of the history of those, Sony was actually losing money on every pair sold ($2,500/pair).
I'm not saying Beats are great, but they get picked on for reasons. It seems almost like a meme and they aren't a terrible product. They work well for what they are meant. People like the way they look. They are supported by one of the most iconic companies in the world and sold in every Apple Store (and many others) with a large variety of different models and colors. Bring them into any Apple Store when something goes wrong and even out of warranty you can sometimes get help from a Apple or a Genius and/or a replacement.