I dont understand why. You can play Korea campaign, then vietnam etc... Just imagine, after 10 battles you will have crew with exp over 100, so after 75 battles it would be ... totally unreal. By the way, you always can buy same composition of troops and manually set exp and names for units...
The point here is to avoid redoing the same composition of troops every time, or manually set things.
Hence my suggestion to "import" an existing core troops to continue with another campaign.
But as I'm the only one interested by this, I'll stop complaining...