– Because we are not enough.
– Because we can’t be filled.
In any case, fast days require an anti-Pelagian caution:
[The dualist view of the soul] is in fact a peculiarly modern view of the matter, not much older than the seventeenth-century philosophy of Descartes. While it is now the model to which most of us habitually revert when talking about the soul — whether we believe in such things or not– it has scant basis in either Christian or Jewish tradition. The “living soul” of scripture is the whole corporeal and spiritual totality of a person whom the breath of God has wakened to life.
David Bentley Hart, “The Soul of a Controversy,” in In the Aftermath: Provocations and Laments (Eerdmans, 2009).