The hair follicles that supply the hair on your brows are different from the follicles in your scalp.
Scalp follicles produce hair at a continuous rate, while the other follicles in your body go through cycles.
The length of them is determined by the length of your cycle, and the length of your cycle is determined by your genes. Which is why everyone has different lengths of hair on their eyebrows, arms, legs, etc...
Drew is right, but there's this thing called terminal height or something, it stops the hair from growing, but the same charestiristic isn't there in your head hair.
not as many hair glans or molecules or something to grow like that. same with pubic hair and other body hair
the hair follicles are smaller