Because cerebral cortex has too many work already. If we have to think about contolling the hair may be we can be only the porcupine instead of two legs mongky. ^^
A characteristic in mammals which enable hairs to stand up, trapping air. This creates an insulation to prevent less heat being loss.
Piloerector Muscle
Makes the hair follicle stand upright
Hair follicle receptors
The structural and cellular features of a hair follicle include the papilla, root sheath, matrix, bulge, and hair fiber. The arrector pili is another hair structure that is a very small muscle fiber attached to the follicle.
The root of each hair follicle is connected to the small arrector pili muscles are small muscles attached to hair follicles in mammals. Each arrector pili muscle has a bundle of smooth muscle fibers which attach to the follicular units which have nerves.
arrector pilli
i assumed you are referring to the arrector pili muscle. usually when you are cold or scared, this muscle contracts, pulling the hair follicle in a upright position. the dents created are called goosebumps. it provides insulation and protects the hair
Standing hair is caused by the tiny muscle that connects the hair to the follicle. This muscle is called the arrector pili.
in the dermis there is the hair shaft, oil and sweat glands, follicle, and muscle.
Each hair shaft grows from a follicle. Each follicle has a muscle associated with it called erector pili (Latin for "hair stander-upper"). When the muscle contracts, it makes the hair stand up. The muscle contracts when stimulated by cold, fear, or aggression.
The contraction or relaxation of the erector pili muscle determines the orientation of hair on the skin. When it contracts, due to fear or cold, hair stands upright.This is a tiny muscle that attaches to the base of a hair follicle mammals at one end and to dermal tissue on the other end.
the hair follicle is a skin organ that produces hair.