I never really considered it, but the SGC is literally like having the front lines of a galactic (and occasionally intergalactic) war placed in the middle of America and the only thing they have defending it is a mechanical iris that’s open by default, operated by a beige IBM desktop, and regularly let’s people through with only a radio broadcast signal that could easily be recorded and then spoofed by anyone assuming they didn’t just take the device itself off the corpse of a soldier and just use it. They have a couple mounted guns and a small platoon of soldiers with p90s at the ready for any gate activations, but this is the only physical force against an enemy which carries personal force fields that effortlessly stops gunfire.

And at one point there was an entirely unguarded second gate that they found entirely by accident despite apparent seismic evidence noticed only afterwards. And the SGC itself is overrun, infiltrated, or nearly destroyed on like a dozen occasions. How the threat barely ever extended past Cheyanne Mountain is beyond me.