Just read it, it wouldn't account for LoS / map design logic. AI just doesn't work that way, it would be more work then it's worth to completely rework AI logic in a given space, also due to the fundamentals of the Harpies, they hover, they don't fly... Much like covenant vehicles in Halo. All in all, the boss fight intended for players to be boots on the ground with the boss, so from an ease of development and incentive to properly use it - what they did makes sense in the entirety.