In addition to what has been said already, in many places the cost to upgrade the electrical service to the building to handle the amount of power that could be generated can be as much or more than all the other costs combined. So now the building operators are looking at millions in cost with a potentially 30 year payback period. It just doesn’t make sense at that point.

Given the egotistical nature of the Goauld I wouldn’t put it past them to have a sort of “gentleman’s agreement” to keep war mostly ground based to help limit the ability of the jaffa to rebel. We never see any Goauld weaponry that can track its target for instance. Everything just shoots in a straight line until it hits something. They want “war” to be up close and messy to help control their subjects. Given that, air defense as we understand it would defeat the purpose of fighting in the first place for them.