Here's why:
* Hyperbole: It's an exaggeration. Nails don't literally pop from the cold.
* Extreme Cold: The phrase implies a temperature so low that it feels like even metal is affected by the cold.
Think of it like saying: "I'm so tired I could sleep for a week." You're not literally going to sleep for a week, but it emphasizes how tired you are.
This expression is often used in folklore or storytelling to emphasize the harshness of winter conditions.