Do we? Perhaps because in those situations we use the cheapest available method to do so? You still haven't given any reason not to use the power generated by the plant to do the repairs.Then why do we always use outside energy sources to repair energy infrastructure?
What reason is there not to do so?
Only if we have multiple energy sources. On Hydro alone we can not power an industrial civilization.
We weren't talking about powering an industrial civilization, we were talking about maintaining a power plant.