The reason they have given is that the text could need to be changed all the way up to release and for scenes that they decide to voice act, they have to be certain it won't change so they choose these scenes carefully. There is also a possibility of them being changed in the future (for example, text changes that explain why the characters are now coming with you as trusts when they didn't used to).

There are games that have no trouble with this, but SE has localization teams to consider and they don't want to be giving work to voice actors unless it's final.