What stops our eyebrows and eyelashes from growing as long as our head hair?

?? I mean it would be odd to see someone with eyelashes down to the shoulders but what stops it from happening.Like when i do my eyebrows and they grow in how do they know to stop growing?
Update: Hey b0iib0ii how about you go get a life you loser, clearly I asked cause i didnt know and it didnt come up on the search, how about you stop answering questions you dont know the answer to for your pathetic 2 points!
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