They aren't, really. Even the nuclear winter is but a hypothesis that could very well turn out to be false.

Nowadays, mechanization and automation make large-scale nuclear first strikes pretty much pointless. Add to that the improvements in missile defense and the result is a different scenario. If we ever see nuclear weapons used in anger again it will probably take the form of artillery used against armored formations, fleets, and satellites.

This might turn a particular theater into a nuclear wasteland and throw global economy back into the 70s, but it wouldn't cause the kind of mass fires needed for nuclear winter.