http://spaceengine.org/manual/making-addons/introduction/
Because from a programming point of view would lead to differences from the default code, and among many consequences, the coder could receive subsequent bug reports impossible to find and/or solve.
That's why SpaceEngineer said that in some future version of SpaceEngine default archives will be validated with some file integrity technique, replaced if needed.
Most of catalog corrections can be done as JackDole explained right now, anyway.