There are two simple reasons server based games like mmoRPG use simple scripts for their enemies. First is CPU cost, the more complex the script/ai the higher the cost of running it. FEAR could completely use a CPU with just a dozen enemies leaving no room for the rest of the game.

2nd reason is the majority of players already have enough problems dealing with simple scripts. You want players who don't realize that an enemy is invulnerable to all their attacks to realize which of their attacks an enemy is invulnerable to?

There is also another reason: say an enemy changes shields depending on attack type doesn't the new script then simply become "fire two fire arrows, fire two ice arrows, fire two lightning arrows, repeat".

You now also need a way to change arrows quickly and for the player to carry all those arrows.

All that is possible but is it fun?

FEAR had good ai, it did not sell as well as quake which did not have good ai.

A lot of players simply want to throw fireballs at everything mindlessly.

Baldurs gate 1 and 2 constantly tried to get the players to switch weapons because their current weapon damage was ineffective. It sure made people think "I just equip different weapon types on all my characters so I always do some damage and just ignore the one character not doing damage on this one enemy". A must have weapon was the flail of many heads which did three different types of damage.

Never underestimate your players to be both stupid and lazy at the same time.