Or, is there something I'm missing?
What you are missing is the history, which explains how we got where we are now with the 9mm Parabellum. (and its competition)
And this also includes the history of handgun ammunition design, and people (customer's) attitudes, and expectations. (I'll skip a lot, and just hit some points)
First off, the original 9mm Luger loading was a 124gr FMJ @ 1050fps from a 4" barrel. This changed shortly before WWI into a 115gr FMJ @ 1150fps from a 4" barrel.
With one huge exception, the "high capacity" of the 9mm over the .45 was two rounds. And this held for many decades.
The exception was the Browning Hi-Power, appearing in 1935, which had a 13 shot magazine. Took more than 30 years before another (fairly) successful design held more. And its wasn't until the 1970s that commercial ammo makers began offering hollow points in handgun ammo.
And it took bullet makers some years to make JHP bullets that performed well enough, often enough to satisfy our expectations.
The wondernine "revolution", then the Glock (and the polymer frame guns that followed) captured the police market, with the US military's switch to the 9mm making that possible.
So with a lot of people (especially police) using the 9mm, (and being "stuck" using the 9mm) when real world results were less than desired, the only place things could be improved was the 9mm ammo.
One thing they knew would improve 9mm performance (while working on better bullets) was to increase the velocity. So, we get 9mm +p
and when that isn't a "magic bullet" some go even further and make 9mm +p+.
Also during this time, other alternatives were developed, like the .40S&W and the .357 SIG. There is a lot of overlap in the timeline.
Today, a civilian has lots of choices, and can easily go to a bigger/different round, if they aren't happy with the 9mm. An organization (such as the police) cannot. Not easily. Many did, anyway. Some have gone back to the 9mm, some haven't, or haven't yet. Part of that is political, part is economic, or a mixture of both. Only some of it is the round's actual performance on the street.
Today, if it doesn't hold 15+ rounds, a lot of people don't think its a good choice for a service pistol. only 9mm does that easily. That wasn't always the case.
That's the way I see it, feel free to disagree...