The biggest reason for that is quite simple - they have vastly more freedom to do those sorts of things when they can simply write a line or a piece of dialogue and stick it in the game immediately.
With voice acting, it goes without saying that you need a voice actor. If you're attempting to add or change character lines after the fact, then you need to bring that voice actor back in, which costs more money. The simpler solution there is to simply not tweak those things and leave them as-is, even if you've thought of something much better.
Without having to worry about voice acting, the writer(s) can largely do whatever the hell they want. Add/remove/change dialogue, modify characters, modify plotlines, etc to their hearts' content.