The problem is that, the father and mother of AR-15, that is Eugene Stoner and Armalite company, had already thrown the DI into the waste basket.
Eugene Stoner's Stoner 63 used the stroke-piston system similar to AK. Armalite AR-15 re-introduce the SVT gas-operated system with rectangular receiver chamber, foldable buttstock thanks to the spring DOES NOT lies in the buttstock.
But the adopted father, Colt, had eaten the shit of propaganda and advertisement, and transmitted the shit into the American public that AR-15 (and the successor M16) has revolutionary design, is the best rifle, the most popular, the weapon of hero against terrorist AK-47... And as a result, the U.S. army has adopted the most hated rifle as its standard weapons.
The famous service rifles of European countries, like FN FAl or HK G-xx, none of them use the DI. None of them used the cylindrical receiver chamber, and most of them do not put the spring into the buttstock to make the buttstock unfoldable (well, GarryB has pointed out that some European rifles put the spring into the buttstock, and I trust him).
Being the slave and vassals of the U.S., European countries have to use U.S. barrel and U.S. standard of catridge, and even G36 has to use th 7 locking lugs bolt of Colt. But none of them accept the DI system.
M16 goes into the history as the most hated gun in the world, and suffer the most humiliating defeat in the history of assault rifles as nobody in Europe accepts it.
Look at around the world, the service rifles of all the countries in this world: ah, systems which are similar to or based on Russian and German ones. American legendary DI ? No.
A good gun has to open fire when the user want it to. M16 legendary DI with legendary choking issues and deforming due to uneven heat ? No.
Meanwhile AK's air vessel's diameter is equal to 2/3 of the caliber of M16. It can't choke.
Look at the reality of Afganistan and Iraq. The U.S. tried its best to equipped M16 versions for the local troops. But then, even the U.S. troops started to hate the M16 and seek for AKs. U.S. has no choice but buying the AK versions of Eastern Europe allies. And since the Eastern Europe allies started to tag along with NATO standard, the U.S. had no choice but ask Russia for AKs. And it is said that 70000 Russian AKs had been made for the U.S. allies in the Middle East.