Since they're using it for wiring harnesses, sounds like these parts are extremely numerous and can add a lot of weight if a cheaper material like aluminium was

Since they're using it for wiring harnesses, sounds like these parts are extremely numerous and can add a lot of weight if a cheaper material like aluminium was used. No doubt they have some pretty strict strength requirements, but they're not holding the plane together per-se. A plastic part would probably work but for issues of fire safety, wear from friction, and other factors like that.