Prevailing culture.

A kid these days in most urban parts would tend to say "911 I have an emergency" while roleplaying because we hear it so much in popular culture.

What else would someone who dials 911 be trying to achieve? And does the benefit outweigh the cost?

911 has enough mindshare that it'd be silly to use it for anything else. And if you're not going to use it for anything, a redirect seems like a very productive way to park it.

Because it's a number used for emergency services in many parts of the world, and not for ordering pizza, plane strikes or whatever.

Relatively speaking, it actually isn't that popular - check the map on https://en.m.wikipedia.org/wiki/List_of_emergency_telephone_...

That's what, 10% of world population? 112 has India going for it…

10% sounds incredibly significant. That's nearly a billion people.

Sure. And in a spherical cow world, assuming you want to make n emergency numbers work, you'd sort by population-using and take the first n.

…but even then 911 would only be an emergency phone number if n ≥ 3; first two places being taken by 112 and 110 (because China).

However, this completely ignores facts like people from China being far less likely to travel to e.g. the US… it's more of a "of the people who'd make emergency calls in XYZ, what numbers would they use" consideration…

…it's really not an easy question. GSM went with 112 and 911 and I guess that's as good as an answer this'll get.