For some reason I’ve always considered stairs to be an indoor thing, although the definitions don’t confirm that thinking. For instance, when I see a collection of steps carved into a rocky cliff or rocks used as steps to provide safe navigation on a hillside I don’t call them stairs. I don’t talk about climbing the stairs to the upper level of a trail. I climb the steps.
A fire escape is another example of a staircase or collection of steps with a formal name. I’d never refer to “the staircase hanging from the side of a building,” for instance, or to someone “taking the steps outside the window during an emergency.” It’s a fire escape. It has a specific purpose. When you hear the phrase you can picture what it is.