Stargate ZPM factories and wartime production security

Why did the Atlantis expedition never find a ZPM-production facility in the city?

A proposed explanation is that the factories were elsewhere or had been moved away from the Wraith, although this remains speculation about the setting.

The absence of an accessible ZPM factory on Atlantis raises a question about the fictional expedition's energy supply. Finding a plant would still leave the need to obtain resources and operate it. One proposed explanation is that production remained elsewhere, possibly in the Milky Way, to keep it beyond the Wraith's reach. The later suggestion is explicitly hypothetical: a defender losing city ships would move such facilities as far from the enemy as possible. Secure production could also give warships much greater access to ZPM power. The example connects a fictional power source to the practical problem of protecting wartime industrial capacity.