WELL, we need the *23* hours preparation time.
If I am active now and want to start a new war, then the game might find an opponent in 3 hours or later; a time when I am already asleep. Then 5 minutes is not enough for me to prepare the war. I log in the next evening, do the planning and send troops.
It would be different if we could do the preparation before or during the search.
It would be different if wars started at defined times. Then we would come out of a war, register for the next cycle and in the same hour we would have the enemy so we can start preparing.