Because you don't need the same provinces to represent various era of history. The importance of regions changed. A region that's the focus of one game may be fairly minor in another. More: the games don't need the same amount of provinces. In CK2 they are feudal holdings. In EU3, they are the subdivisions of a state. In Victoria, they are economic regions. Victoria especially need more province, while I would argue that a 50% increase in EU3 would make the game tedious, by giving any sizeable countries far too many provinces to manage, to the point of overwhelming the player.
More, and still worse: it's already nearly impossible to divide the map into provinces that fit the entire EU era, and now you want them to split it into provinces that fit everything from 1066 to 1945? GIven how much borders have evolved, even with approximations, that's well-nigh impossible.
Besides which, this would require a Crusader Kings map of the whole world. That's a ludicrous undertaking - what, are we talking about researching polities for the entire world, including the Americas, in 1066 AD? Or are we instead looking at making all those regions empty, and completely outside the game - just a graphical waste of map resources?
Your proposal would do a number on gameplay and performance, and all for what? Easier to build converters?
A) It is not tedious at all. Try the More Provinces Mod sometime.
B) Games don't need identical province shapes/borders. They don't even need the same province names to be convertible. They just need to retain the same province color ID. There, now you can make 4 different maps for CK2, EU4, Vicky2 and a future HOI4 that are completely convertible.