Why do head hair grow without boundaries, and any other hair, eyebrows, eyelashes, pubic hair, hair on legs or arms only grew to a certain length and once you shave them they will grow back to their original length and not get any longer, unlike head hair which will grow and grow unbounded?
Hair doesn't have living cells, it can not know how long it is.
So it is literally impossible for your body hair to stop growing when it reaches a certain length, because it doesn't know how long it is.
Now there are two options for your hair to stop at a certain length.
1: it stops growing after a certain time has past and will remain at that length, until it falls out and is renewed.
2: It constantly grows, but it falls out really quick, never allowing it to grow to long.
If you shave off your eyebrows, they will instantly grow back, so it is safe to assume that they grow at a constant speed and not stop after a certain amount of time has passed.
Therefore option 2 is the only viable option
It does have boundaries, but the anagen phase of scalp hair is much longer (years) than on the face or arms, etc (weeks).
People seem to assume pic related sort of stuff doesn't happen in humans, idk hair is p. sexy right.