because composite materials are often times just as comparable to other materials in strength and longevity. these are engineered products, its not just some sticks. of course you already knew that. it's also about design, if the focus is more internal volume then using composite materials benefits you. also r value is easier to achieve in the colder overall north american climate, likewise in hot areas for cooling.  methodology. being pedantic over the materials is nigger behavior.
United States