“Had we had a prophet like Moshe, we too would have accepted the Torah.” So, to preempt that defense, HKBH gave prophetic powers to Moshe.
Wolf, the wording Chazal use is PROPHET – NOT LEADER. The goyim would have an excuse for not accepting the Torah if they had no prophet to tell them the truth. The issue here is not about leading.
If the goyim would not acknowledge the truth coming from their prophet and accept the Torah then why do they need a leader to lead them in their service of Hashem?