You say that, but there will be a document somewhere telling people what the requirement is, and it will probably NOT say "Imagine the maximum possible amount of people that could physically fit, now add ten percent". And you might even find you need to track down the rationale in the engineering paperwork to know if the structural engineer envisioned this as a route crammed full of people or just a fun decorative element of the building that would technically be open to the public but rarely used. Not everything, build everywhere has to obey the same loading rules, many private homes have stairways that would be far too narrow, steep and poorly lit to be legal in commercial buildings for example.
When there was a balcony collapse in a foreign country I got interested and read the code where I live, the building regulations specify that a balcony or similar structure, which essentially hangs off the side of a building and so is unsupported at one side, must handle up to a certain load per square metre. Enough that a modest residential balcony can certainly hold two lovers, someone's spare bicycle and the decorative concrete flower beds they've put down to make it feel "outside". But what if instead of two lovers standing under the moonlight it's as many of the university rugby club as will physically fit? Ah, well then you're going to exceed the designed capacity and a code-compliant balcony might very well collapse.
All those tower blocks in England that are being retro-fitted because of the ones that burned down in London, their architects would have told you at the time they were definitely built to code, no extraordinary risk of fire. Then a bunch of people die in an out-of-control tower fire and - surprise - the politicians now claim the code meant you shouldn't ever have done that, and those towers were all built wrong but now they need to "clarify" the meaning. Sure.