Save games hold a huge amount of information that the map editor doesn't know about. For example units walking between tiles, serfs carrying things, arrows flying through the air, which unit lives in each house, exactly how old trees are, how hungry units are, how injured units are, etc.
So if did let you edit save games then things will be quite different after you edit it. Units will not have a home or a task, everything will be reset. Basically you would only get stuff like: the terrain, the location of houses and units, and the wares inside houses. So it wouldn't really be a save editor. I don't see the point in such a feature to be honest.
If that is how KaM works, then I understand your point. In Stronghold, for example, I can edit a saved game and do the following:
- Add or destroy buildings -- when this is done, the units assigned to those buildings either appear or disappear when you load the edited save.
- Change terrain
- Add or destroy plants and wildlife
- Add or eliminate various resources -- stone, iron, wood, food, weapons, etc.
- Add or eliminate various troops
- Change the game plan -- enemy numbers, types, and attack timing, natural disasters, good events, etc.
- Create new scenarios that start with partially or fully-built environments of many types. (This is commonly done by fans to create new scenarios for everyone to play.)
- And many other possibilities.
When the new scenario is loaded, all existing activities continue from where they left off in the saved game. Units continue walking to their destination, arrows continue to fly, trees keep growing, etc.
Now, I understand that KaM is not Stronghold, and I don't expect it to be. They are very different games, and are programmed differently. I merely am pointing out that there are many interesting things one can do by editing saved games. If this is not possible in KaM, or would require too many changes, no problem. KaM has its own strengths that make it a fascinating game.