Id guess that the reason it doesnt exit because 99% of the population would not choose to do this. There are lots of things to work on, this seems like a low priority imo.
Maybe it doesn't concern many players, but at the same time, Larian really wants to create a RPG that gives players freedom. Also, it doesn't sound like too much coding for the goblins-vs-grove arc to have a variable that detects if the grove was massacred and if consequently a future conversation (with Minthara) will make no sense and lead to a dead game.
I think the main reason why it's not done yet is that it's Early Access and Larian hasn't tied all lose ends on this arc yet (I want to assume that they are competent enough to have seen that there might be an issue if players choose to kill everyone in the grove).
I'm in the vast majority of people for whom this won't be a problem (I have more evil characters than good characters lined-up, and none is a murder-hobo), but really, it sounds quick to fix and it would be a blatant bug if the game is released without this fixed.