Hair

It's often the facts that stare at you in the face, that are the ones that are overlooked. All human hair has a terminal length. Once it's grown for certain amount of time and reached a certain length it'll have served its purpose and fall out. Head hair can obviously grow much longer.

Hair growth is a continuous process where the maximum length is determined by the equilibrium point where hair growth and hair loss cancel out.

This could after reaching back length, this could also be ankle length hair. It mostly just depends on generics.

This process can be summarized as hair having a growth phase, a cut off nutrient phase, and finally, a phase where the hair falls out or is pushed out by new hair. Head hair has a growth phase (anagen) of a few years, whereas body hair only has an anagen phase of a few months.

Thus explaining the huge difference in max length.