Actually the AI isn't 'good' but they are more of a threat than players in masses. Due to wings of 8 sidewinders being way harder to kill than a well fitted player Python, AIs are more effective than players because of the following reaons:
1. They exist in masses. It is a rare thing to meet another commander in the current version of Elite and if you are meeting one, then it is only
one.
2. Suicide attacks. A player will most likely run if a Python interdicts a Cobra (the Cobra player will run). NPCs will most likely attack and fight to their end causing signifcantly more damage and weaken the player for a short period of time.
3. Due to 2. the following NPCs have an easy way to kill the player. While the total KDR of NPCs vs Player is probably something around 10:1, NPCs don't care if they die, they just respawn and have no insurance cost and stuff like that and so they behave, they behave like they have nothing to lose and this is the problem and the reason why AIs are considered as good.
NPCs just have a major advantage that they will use, causing them to be an actual threat. But please, don't tell me an NPC is even close to being good as a player (though, AI could be made to be UBER-SUPER-MEGA-OP

)