Why do we not need to cut our eyebrow hair?

I trim my eybrow hair from time to time, because they get too bushy.

But basically eyebrow and body hair grows more slowly and falls out and gets replaced more often than scalp hair, so it never grows very long.

Same with most animal fur.

You don't actually need to cut or trim any hair. People only do it for cosmetic reasons.

One of the big aging differences I've noticed is that my eyebrow hair grows longer.

Some of us do.

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Same reason we don't need to cut our arm hair, leg hair, or other hair. It grows to a length that we're comfortable with.

Head hair grows on its own to a length that we're not comfortable with generally, and cut it before it reaches that length.

Hair on your head is the only hair that needs to be cut to stop growing. Eye lashes, leg hair, arm hair, armpit hair, pubes etc all so growing at a certain length. Hair on different parts of your body just know how long to grow to. Your head is the one exception.