Because SAG is Bina, and it has the quality of 2 as its main quality; so when the Light is expelled, on the way out it reaches the Reshimo of 2/2 within the general quality of Bina, and only then is it truly the pure quality of Bina, bestowal, which has no desire for itself and can truly be anywhere.
We can’t say that it’s planned this way by the Kli, as here we are talking about a process that the Upper Light is moving the Desire through. But that outcome you describe is, of course, in the overall Plan of Creation.
We’re actually in the spiritual world but we have such senses through which we see it as we do. So, it doesn’t appear as a beautiful, perfect, infinite reality of all good and no bad; rather one who is realistic sees exactly the opposite.
I may not think I have free will, but no matter what anyone says, I still behave as though I do. Because if one feels how in every thought, desire and movement he is being operated by a force that wants bad for him (the ego), then he would search for a way to stop following these dictates. Baal HaSulam explains free choice in the article, The Freedom https://kabbalahmedia.info/sources/4AtF9tGS