Secondary shower thought, an image of Captain Caveman should be the image representing humans on the Wikipedia page.
Tertiary shower thought, we probably started wearing clothes because our own hair used to be our clothes, and we wanted to replace the hair we cut off when we started grooming.
It’s not that hair knows it’s length and stops growing until you cut it, there is no mechanism for it to register its own length. Rather, there is a natural rate at which hairs will break or fall out and begin to grow again, and so a person’s hair can only get so long before the longest hairs are due to be culled.