Why cannot a single silk fiber form a thread?

A single silk fiber is too thin and delicate to function effectively as a thread on its own. While it can be very strong, its lack of bulk and structure means it isn't suitable for the tensile demands of sewing or weaving. Additionally, multiple fibers are needed to provide the necessary thickness, durability, and flexibility for practical applications. Combining several silk fibers into a thread enhances their collective strength and usability.