It’s for the species. With age comes wisdom and panache. If that wasn’t accompanied by gross ear hair, the young studs would never get any.
The total length any strand of hair reaches is a function of two variables: the rate of growth and the length of time it grows before falling out naturally. As with any physical trait found in a large population of individuals, there will be variation within the population for both of these variables. We notice the individuals that are on the far extreme of the bell curve.
QFT.
I’ve heard it was a given amount of time.
CookingWithGas already spoke to it, but I have to say that my hair is very light and I’ve still never been able to get it to grow past my shoulders. You’re right about protected hair growing a little longer, though. For a couple of years I kept it braided and got maybe an extra inch and a half.