Why does arm hair stay short, while the hair on the head grows very long?

Each hair on a body grows from its own individual hair follicle. Inside that follicle, new hair cells form at the hair shaft's root. As cells form, they push older cells out of the follicle. When the old cells are pushed out, the cells die. We see those dead cells as hair.

A hair follicle will produce new cells for a specific period of time depending on where on the body it is located. This period is called the growth phase. Hair has a growth phase, a rest phase (when it stops growing temporarily), then will restart the growth phase again.

When a hair follicle starts its rest phase, the hair shaft will break. The existing hair falls out and is replaced by a new hair. Therefore, the length of time the hair is able to spend in a growing phase controls the maximum length of that hair.

Cells that make the hairs on arms are programmed so they will stop growing every couple of months. The hair on your arms, therefore, stays short. The hair follicles on your head, however, are programmed to let hair grow for years. So that hair can grow very long.

Shedding animals have hair follicles that synchronize their rest phases so all the follicles enter the rest phase at once. That way, all the hair falls out in the same period of time. A shedding dog will lose hair in large clumps.