I'd like to point out, as Robtard alluded to, that there are TONS of nukes in Skynet's arsenal.
An often quoted phrase, "there are more than enough nukes to destroy the world 7 times over." That would turn the entire planet into a radioactive Wasteland...even more severe than in the game Fallout 3. Because much of the earth is inhabitable in Salvation, we know for a fact that very few nukes were used. That leaves hundreds if not thousands of nukes still in the possession of Skynet.
On top of that, PIS is off: each side uses all of their powers to beat the other.
There are between 27,000 and 28,000 nuclear weapons. Damage radius yields would be between 3.5 and 17.5 miles.
Find the area of the 'low-end': 38.5 square miles. Multiply that by the number of nukes, low-end: 1,039,081.8 square miles.
Square mileage of earth's land: 148,940,000 km2 = 57,506,055.6 square miles. Or about 55 times the land area destroyed by the low-end estimate.
But what about the high end?
One bomb would have a damage radius of 962.1 square miles.
That's 26,939,157.0 square miles for the 28,000 nukes. Or about 2.1 times the land area.
The nuclear fallout from either end of the scenario would make almost every square on earth irradiated and unlivable. This is how we know that no human would survive (after the first day, pretty much every last inch of the surface of the earth would be covered by 500 or more rads) and 400 rads in one hour is a "fatal dose." 4500 rads in an "acute" period is a lethal dose.
What was I talking about, again? Oh yeah, nukes.
So, yeah, since tons of the planet is still inhabitable, they did not even come close to using up all of the nukes.