The nut we eat is only a small part of the cashew fruit, the cashew fruit is a much larger part of the whole that probably takes more of the nutrients of the whole product. The fruit is not marketed in the west and a lot of the actual product goes to waste, you're paying for a part that you didnt even know existed. I also imagine the high fat content also gives it a shorter shelf life from tree to shelf, and because the nut is on the larger side it is prone to breakage which would mean only the whole nut can be sold as premium product the broken bits are sold as toppings or used to make cashew milk or butter.
I believe the fruit can also be eaten or used to make wine