because it's stupid. Yamato was as big you can get a battleship. Montana was at the limit and thus was one of the many reason they was not build. (and also they will come to late to be useful)
Off of what principle? The Montana could be no larger than designed due to the size of the Panama Canal and was dropped due to the importance of carriers and their slow speed. As of Yamato, the design could have been much larger, and the Japanese navy had commissioned such drawings yet suspended the project due to their carrier losses. A battleship can be as large as needed under the conditions of a suitable roll, funding, materials to construct, and the size of the construction way.