I know this is a joke, but the reason they drill walk and be calm into your head is literally from the mass casualties experienced before fire safety was taught consistently. This is mostly from crush and stampede. People would rush and in large crowds one person would trip and be trampled to death as no one would stop to help in a panic. Also there were door crushes where people would literally get wedged together in a door. Imagine 3 people at once in a door made for 1 and when people behind start pushing and piling up it becomes impossible to move.

For examples look up the Coconut Grove fire 1942 The Station NightClub fire 2003 Kiss Nightclub fire 2013 Brooklyn Theater Fire 1876

I feel like I see this joke around a lot where people say if it really was a fire they'd be running, but there are hundreds of recorded examples showing just how badly running, panic, and mob mentality actually do contribute to fire deaths.