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.

Thanks for saying this. None of the people here have even tried the headphones yet, don’t know what they are made of, and don’t know how computational audio plays with material..

I own a pair of midrange Dali speakers which use compressed paper cones and sound really great.

You need some material which is rigid enough to push the sound waves in the air, but light enough it can be pushed to high frequencies. If plastic works, why not?