“And We gave Moses the Book and made it guidance for the Children of Israel, [commanding:] ‘Do not take anyone besides Me as a guardian (wakīl).’” (Qur’an 17:2)
Allah forbade taking anyone besides Him as a guardian because a created being cannot independently fulfill all of a servant’s needs. Permissible agency (wakālah) is when a person entrusts another with a task that the latter is capable of performing, thereby fulfilling some of the one who appoints him’s needs. As for fulfilling all needs, none is capable of that except Allah alone.
— Ibn Taymiyyah (Majmūʿ al-Fatāwā 4/202)