If divine will were originated and its subject were God's essence, a new attribute or state would be made to subsist in that essence. Within the theology under discussion, the occurrence of originated states in the divine essence conflicts with the eternity and transcendence of divine attributes.
Human willing may begin after a person previously did not will. Transferring that sequence directly to God erases the creator–creature distinction.
The question is therefore not simply when an act occurs, but whether the act must be explained by a newly arising attribute in God.