Maybe it's lighter than the alternative? Not everyone wants to wear a brick on their head.
But just the word "plastic" doesn't really give any useful information. With all of the different formulations of plastics out there, the physical properties span an extremely wide range.
Source: I work in the polymers/materials industry.
> Mainstream taste in audio is crap. Designing headphones for a target audience that thinks Spotify HQ mode is sufficient is not going to produce a very good product, audio-quality-wise.
Mainsteam taste is what sells products. Obviously you're not part of the target market and should probably just move on.