Not being a musician, I had not been familiar with instrument cases, specifically for guitars. Many of them are made of composite materials, both fiberglass and carbon fiber.
The advantages of composites align well with requirements for handling musical instruments. Composites are strong and lightweight, as well as impact resistant. Composites are also not affected by moisture.
I came across an interesting video by JeffHoffee who does a test of his Hoffee Guitar Case.