If kabuto doesn't fight in a cave then his moves are weak. If you look at all his moves and think about the fundamentals of his moves property then you could summize how they would work in an open field. P.s. all his edos were beaten already except for madara so he aint calling his edo army to back him up. Like I already said if Itachi wanted to kill him he would be 6 ft under but instead he wanted to end edo tensei and save kabuto from himself. Thus, using Izanami was necessary. Also it would have been much easier to trap him in izanami outside the cave.
This is just a question I always had, How do you count edo tensei as part of kabuto's power when it requires the most prep time and the edos are clearly are able to fight against this technique. Ex. Sai's brother, sasori, Itachi, hanzo, madara, third raikage and mizukage( they were telling the enemy about their weakness), Asuma(talking to his team during their fight).