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A huge reason is that you have to cut the roster down. Spring training isn't simply a try out, it's mostly about getting players ready for the season. That means the guys who are going to make the team need to get plenty of work in so that they do as well as possible. You can't have 60+ players in camp right up until you leave for Japan. So if the decision has been made that the young pitchers will start out in the minors, you might as well cut them now so that Millwood, Iwakuma and the relievers get plenty of innings so they are sharp.