Because each follicle grows for a fixed stretch of time before it rests, and that stretch is genetic. Growth speed multiplied by that duration gives you a maximum length. For some people that is past the waist; for others it is the shoulder. It is not breakage and it is not your shampoo.
Transcript
If your hair seems to stop at a certain length no matter what you do, it isn't breakage and it isn't your shampoo. Each follicle grows for a fixed stretch of time before it rests, and that stretch is genetic. Growth speed times that duration gives you a maximum length. For some people, that's past the waist. For others, it's the shoulder. Nothing in a bottle changes the number.
Nothing in a bottle changes that number. What products can do is reduce breakage so you actually keep the length you grow — hair that snaps mid-shaft never reaches its own limit.