We've discussed this about 8 million times.
1. Cliche - if you can't do the job with 5 rounds or one mag, you ain't a manly man.
2. Folks say the average gun fight is 2 to 5 rounds and thus assume it will always be two to five rounds. Go to stat class.
3. Folks only plan for the one mugger be gone scenario.
4. I don't go to bad places. This assumes that a gun fight in a nice place will be nice. They assume that you only need more ammo if you go to Zombie land, a slum or urban riot. In a nice play, you don't need more ammo as the bad guys are nice.
Other viewpoints:
1. While rarer, you can get into some intense critical incident where the more rounds the better. Of course, then we will get the folks who think they are totally helpless in something like San Bernadino. If they think that, they probably are.
2. An extra mag with a semi is as much for malfunctions as for extra ammo. Oh, someone will say that they shoot their Tarum or Gluger 500 rounds and it never jammed. Shoot enough - everything jams. Magazines go bad. Mags drop out of guns when you don't notice that the release got pressed. Oh, it won't happen to you. A friend of mine dropped a 1911 mag and when it hit, it unloaded all the rounds like a zipper.
3. You plan for not the average but a reasonable cut off on a incident intensity continuum. It's like the significance level in hypothesis testing or other risk analyses. Most folks think a semi and an extra mag is a good cut point for EDC.