Because it is a video game and taking agency away from the player via NPC party members messing up a players plan would likely cause more overall rage instead of enjoyment; This is a isometric RPG, not a TT-RPG. Also as a side note, there is a limit to what party members tolerate. I have turned party members against me and they tried to attack me. Astarion for example turned hostile after I made sweet love to Volo's chest with the business end of my broadsword.
