Fire escapes stairs are relics of a bygone era. You will notice that you never see a new building with a fire escape stair. I'm not aware of any municipality that still allows for their construction of new buildings.

Fire escape stairs were constructed to provide, as the name implies, egress in the case of a fire or other non-specific disaster. Because buildings are prohibitively expensive and have historical value, we allow older buildings with fire escapes to be "grandfathered in" despite the fact that the fire escapes do not meet current design codes. Similar exemptions are allowed for elevators, staircase dimensions, the presence of or lack of wheelchair-accessible ramps, etc.

Modern building codes require egress staircases to be placed internally in a fire-rated space. The exact rules will vary by building and municipality, but in the US, you will generally find that egress stairs are isolated from the main building space. These are not the fancy glass staircase leading to the mezzanine of your designer venture capital office. Egress stairs are the ugly steel stairs encased in a vertical concrete shaft with heavy metal doors at each floor. These stairs are designed to not have flammable materials, to not allow the fire to spread up through the shaft, and finally, the stairs and walls are designed to withstand a specific amount of heat for a specified time, for an egress stair, this is typically what is known as a 4-hour rating.

So in theory, if a fire is burning outside the stair for 4 hours, the fire escape stair within should still be functional for someone who needs to escape the building. There are actually specific prescriptive tests for fire ratings of various materials, generally, Architects are your best source for information on this subject.

In some municipalities, the requirements for egress stairs are allowed to be modified or waived by providing specific alternatives, such as active fire suppression systems, using water sprinklers or inert gas.

Fire escapes are not as easy to climb up or down as they appear from the ground. They typically contain large gaps in the railings and stair treads that, when combined with the great height, can make the experience very stressful and dangerous for some individuals. Furthermore, because fire escapes are external and exposed to the elements regular inspections are required to ensure they are in working order. On some inspections, I would find large playing card-sized pieces of corroded steel that would flake off the metal framing with a flick of the wrist.

Finally, fire escapes stairs also posed a potential safety risk for occupants of the building as they gave criminals a route, often from a dark alley, directly up to the various floors of the building. This is why the bottom stair is typically operated by a balanced torsion bar hinge, meaning it doesn't swing down until sufficient weight is applied from above, however, this is obviously not a foolproof system.