Not to mention that Kishi didn't want Itachi to succed in taking Naruto.
Normally it would be stupid you are right.
But it just happened for the sake of the story. We can't argue about something like that.
Look at these examples. Why did Rukia loose to a normal weak hollow in the first chapters of Bleach, when she could've easily slain it?
Because if she had slain him, there would be no shinigami Ichigo and no Bleach at all.
Or why didn't Raito (deathnote) kill all in the team that investigated on Kira after he had finished L off. He didn't need them anymore but kept them alive for no reason for over 3 years.
The answer. They were need for the second arc after L died.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"