I'm not sure I understand why NPCs are prevented from interacting with the environment while you're talking... given that gameplay isn't stopped. Seems like an arbitrary decision that might lead to other surprising behavior too... e.g. talking to someone else to interrupt an NPC performing a scripted action.
Probably because interacting with the environment could cause environmental changes that might affect you.
For example, if you're standing in a doorway talking to someone and an NPC wants to close the door, it could close in between you and the NPC you're talking to.
I seem to remember a fallout clip where the character is trying to have a conversation with the NPC but then a monster is also attacking the NPC or something and the NPC is just oblivious. Kind of breaks the immersion when you are having a normal conversation and the NPC is taking mortal wounds at the same time.
It would be hard to make believable interactions when unexpected environmental side effects can happen at any time
