Because governments are inefficient. Decisions are many times made for political, rather than functional, reasons.
That’s why the original M-16 was abhored by the soldiers who had to use it.
After the US experience trying to kill Phillipinos with .32 revolvers, the army demanded an effective “defensive side arm” which we know now as the 1911 .45 caliber pistol.
Even today, the “Southern Command” and special forces rifle isn’t an M-16 with its itty-bitty bullet, it’s a .308 called the SoCom-16. In the 1980s the special forces also demanded that they be allowed to pick their own pistols, and imediately went back to the 1911.
The American use of the 9mm was a political decision, because it was easier to standardize NATO on the 9mm that “everybody else” was using, than try to convince the hoplophobic Europians to arm mere soldiers with the .45.
Keep in mind that the reason the Dumdum ammunition company started making soft-point bullets was because soldiers were complaining that the full-metal-jacket rifle bullets were merely punching little holes in the enemy, rather than killing them the way the old soft Minne-balls would. Oh, but that is so politically incorrect that the “humane” governments all poo-pooed the idea. “That would be barbaric!”
Thus the political overcomes the practical.
But in response to the under-powered round, civilian makers realized they could pack a whole bunch of 9mm rounds into the space of a pistol, so the “wonder 9” was invented with 12 and 15 rounds, and even 30 round magazines which fit into the pistol magazine well. Keep one full in the glove compartment, just in case you have to reload.
“But that’s barbaric!” went the politicians, and the 10-round magazine limit was created.
The response? The civilian mass-market 10-round .45 pistol, loaded with high-power soft-point and hollow-point ammunition. Serious punch. They’re harder to find since the 10-round magazine ban sunsetted, but I’m sure they’re around somewhere.
I love the free market.