Ether 2
Notice that the Lord does not gather all the righteous together at all times. He had the city of Salem where the people were righteous at the same time as Abraham (and probably at the same time as the Jaredites leaving the tower) but He did not have them come together. he spread each little group out to act as a leaven among the people of the world. he gathers his people when they need to draw strength from each other and he spreads them out when they are strong enough to be self-sustaining in their faithful covenant keeping.
When the brother of Jared asks about light he would have accepted the decree of the lord if the Lord had said that yes, they were to cross the ocean in darkness.