The most basic answer to your question is that "it's just not how we say it" and, in fact, our use can mean something quite different.
For example, if someone said "Her hair was falling around her face in ringlets", this would normally mean that she had curly hair that was being worn down and framing her face. Something like this:
[image from here]
It would not mean that her hair was falling out of her head, the way you might describe a leaf falling off a tree.
Additionally, you don't want the shampoo for the hair that is falling out of your head. Certainly you don't mean to wash the hair that is no longer attached to your scalp?
Instead, you want shampoo to prevent your hair loss. So, you would be better to say something like
I want shampoo that will prevent me from losing any more hair.
I want shampoo that prevents/reduces hair loss.
I want shampoo that stops my hair from falling out.