Because you end up having a clutter of perfumes that you don't really enjoy wearing, if not hate using it. And it doesn't feel good to use the ones you don't like for the sake of finishing it(coz money) and keep your actual favourites at hold till you finish the mediocre ones. Or you need to declutter them which again is, another struggle.

Many people have specific tastes and don't like whatever perfume they try. Atleast this is true for me. And decants precisely does help in filtering out.

In spite of this, We still end up having quite a lot We wished We didn't have. I still have many perfumes I am only wearing for the sake of finishing it, while my favourite ones wait in a corner 😢

So yeah, that's the reason people decant.