Another reason could be that music and/or sound effects overlap with her dialogue making it difficult to clean or edit. It would be a LOT easier to record a soundalike than to try to edit/filter out the dialogue SFX overlapping the original track. Theoretically, they could, but the cuts would sound out-of-place with the background noise in there. Trust me, coming from someone who has done post sound on some *smaller* films, it’s difficult.