There’s lots of reasons for that.
1. AK sights suck and the monkeys using them don’t know jack
2. AK rounds won’t penetrate balistic armor vests
3. AKs are typically fired full auto and you can’t hit anything that way...not that it matters since even if you hit a US soldier with a vest on, it won’t kill him.
4. AR sights are kickass and the monkeys using them DO know jack
5. AR rounds will penetrate a vest, although the monkeys they shoot don’t typically wear a vest
6. ARs are fired single fire and are famous for one shot kills...usually head shots.
lol....