It's about using the harpies' Creator-Given combat mentality and tactics.
Imagine running into a pack of wolves that didn't attack as a pack. That's what most of the creatures are like in BG3. They don't behave as their creature race says they should.
That is exactly what im talking about ... Imagine you are intelligent (or at least sentient) creature with both wings and hands ... with opposite thumb ... so you are totally capable of graping and throwing stuff.
What reason in every Hell you would have to "swoop down and fight in meele" while your greater advantage over litteraly everything except other winged creature with both wings and hands ... is that you are totally capable to fight from safe distance? Especialy since you are living on such place as this rocky cliff, that is litteraly FULL of potential amunution?
//Edit: I would understand the idea: Try to charm them ... slit throats of those who are charmed, before they wake up ... But anything else is willingly put yourself in danger ... for what benefit exactly?
Throwing stones. 1d4+1 damage (5 max), or at least it SHOULD be, but Larian has them dealing up to 8 damage per stone somehow (based on last playthrough). Regardless, slashing with claws and bashing with club. 14 HP potential damage (2d4+1 and 1d4+1). The entire tactical advantage of harpies is Luring Song, Charm Targets, swarm them, kill them fast.
But yes. IF that tactic fails and their targets start to win the fight, THEN it makes sense to maybe have them start to take tactical advantage of flying away and hurling stones.
