Because it does not work the way that many people think it works.
"Manifesting money" is only a very small part. Manifesting prosperity is very different. If a person started off with a poverty mentality, then they'll still be poor regardless of how much money they have, and they'll often spend all the money the moment they have it. This is part of scarcity thinking; I've been cash-poor at certain times in my life and there was the tendency to think that you'd better go out and have a shopping spree *now* because you might not see that money in one place again.
Things like money and relationships still have a learning curve.
I look back and realize that my broken relationship that I ended two years ago, was part of my attempt to manifest relationships before it started... because I could not have a happy relationship without the things I learned in that relationship.
"Manifesting" isn't just a means to an end, it's a whole path and life orientation, and if a person doesn't grasp that, they'll be inclined to lose any wins they make. It also means that you are getting the lessons so that you can achieve those things more easily. It doesn't necessarily mean that things fall out of the sky.