Why is it that your beard and head hair keeps growing but all other body hair seems to stop at a certain length? I did do a search on this but the several thousand results i got back didn’t look like fun sifting through, although i imagine this is a common question.
The short answer is - no one knows why.
The long answer is not everyone has facial or body hair (white folks are furriest) and even head hair grows to different lengths on different people. There are some pretty good arguements for why people have eyebrows and eyelashes and pubic hair and ear and noise hair… but plenty of people dont’ have much in theway of eyebrows, eyelashes and pubic hair and those ethnic groups did okay without 'em.
The above discussion of fur-bearing folks assumes a baseline of health and regular trips to the feeding trough. Illness, aging, and environmental issues can cause hair to fall out or change texture or stop growing … etc.
http://www.straightdope.com/classics/a1_097b.html
Cecil’s take on the subject.