If I didn't know the noun compound "hair loss" and I wanted to describe to a pharmacist that I have hair that has been falling out and needed a shampoo with the right formula for it.
Why can't I say " I need a shampoo for my falling hair"?
I am given to understand that you can say "the falling leaves" and "the falling snow" Why can't you use this adjective for hair and describe a condition in which you hair has been falling out?
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.
The "falling leaves" or "falling snow" represent the elements (precipitation of sorts) that is not attached anymore to its source. The leaves are not on the trees, the water droplets are not in their clouds.
The same with hair. What's the point of washing the "falling hair"? That pair of words represent the hairs that have already detached themselves from their follicles. Do you really need a shampoo for that?
You admire, ruminate over, then rake falling leaves. You curse, dance under, falling snow. You usually simply forget about falling hair, once you remove it from your comb or brush. What you fret over is the remaining hair, the hair that is now thinning (while still attached to your scalp).
Generally I would say "fall out" since hair is falling out of your head.
Since hair loss is a condition but you don't know this phrase, it takes some more words to try to say the same thing.
I need a shampoo that will help keep my hair from falling out.
This a bit different from "the falling leaves" or "the falling snow", though they can both fall out of the sky.
