You do realize that BETTER WEAPONS that are suited to the SMG role actually do exist and are fully utilized throughout the world?
Sig, HK, CZ are just three of many more manufactures that have dedicated SMGs for pistol caliber rounds.
CZ just released a civvy version of this the Scorpion. CZ is working to get approval to allow people to SBR it... which would be so badass... I can feel the drain on my wallet now....
Also having built two AR15s for myself there are parts that aren't compatible with a .223 AR such as the lower receiver (unless you buy a magwell adapter like some scrublord), upper receiver (the shell deflector needs to be moved for 9mm), bolt carrier group and buffer. The barrel and muzzy device are a given.
In the guns world, "better" is often a subjective term.
Many people favor the AR-15's SMG variant for one, very big reason:
Familiarity.
The AR-15 platform and most of its derivatives is one of the most well-known and often used gun platforms. At very least in the U.S, if not across several other countries.
What is easier? Teaching an entire division of the army how to hold, fire, arm, reload, disassemble, clean, fix and reassemble an entirely new weapons platform? Or just handing them something they already know inside and out and saying "Here, it's like the other gun, but with a smaller cartridge."
Military and enforcement and, well, anybody who uses firearms in a professional sense needs to know more than the basic "point it here and squeeze the trigger" bit. Any idiot can pick up a gun and shoot it with very little training. But to actually maintain and care for a firearm? To know what to do with it if you're on the field and suddenly it stops working? To know how to safely handle it and how to strip it down, or build it back up? That kind of stuff requires training. Lots of it.
I know that, at least here in Australia, our servicemen are expected to be able to disassemble, check, and reassemble their service rifles (which, for the record, look like
this) while
blindfolded. And they get timed doing it, to boot.
If you're a commanding officer in charge of handing out new weapons to your army, you can't just palm off something they've never used before and say "go wild, lads!" If the option is there to give them what they need (and yes, sometimes, they do specifically
need a 9mm cartridge in a carbine setup) then you damn well take that option because that's several weeks and a whole bucketload of money saved on time and training.