Minecraft is laggy enough as it is when it rains, when you're near a lot of Lava, or when you have 'complex' redstone machines running (to name a few).... can you imagine how much more laggy on a server it'll be if everyone builds large sail ships to sail the seas ??
The Meaning of Life, the Universe, and Everything.
As the game is coded, blocks moving would look really stuttery and if not implemented correctly could produce tons of lag. They should use entities like boats and minecarts, which could have code used for these larger ships. Some code could allow you to freely walk on entities and even move along with them. It would work better and could be considered more minecrafty than having a bunch of cobblestone and plank textures floating around.
A larger type of crafting table could be used to construct these ships.
Those mods exist because they literally wrote all the underlying functions that make it possible to do. Minecraft itself is completely unequipped to handle this concept, and while that just means more work in the development phase to make it happen Mojang has indicated they would prefer not to add this to their game.