Why does hair stop growing at a certain lenght?

The length to which you can grow your hair is determined by your individual anagen cycle. The hair cycles trough 3 different phases. The anagen phase is where the hair is actively growing. The next phase is where the hair stops growing and rests before transitioning to the telogen phase. It then falls out and a new hair forms from the same follicle. Someone who has an anagen phase of say, three years will never be able to grow their hair mid-back. Whereas someone with an anagen phase of 20 years can basically make shoes out of the length.