Why AI is good at shooting long range target instead of closed one?

[msy]

I want to show a situation:
You put an soldier 10 meters away from you, and you are at his back. After you fire at his side but not shoot him, he will first go prone and then turn around but he is not aiming at your direction. he will spend seconds to notice you then shoot you.
Another is that you move slowly to approach him, no matter how you move, when the distance between him and you is less than 10m, the enemy will soon find your existance and turn around to shoot you very quickly.
It is very unhappy to see these things.