Yaarel
🇮🇱 🇺🇦 He-Mage
A Dragon dropping Large boulders would only hit 1 to 3 soldiers at a time, or not much more for a Huge boulder if the Dragon is able to Carry it aloft.Dragons generally have intelligence and wisdom equal or better than most humanoids. Why would they let them get pelted by hundreds of arrows when they could do the spoiler?
Breath weapons might be limited to distances similar to bow range, a rock a cow or even half of a cow is not faced with those limits however. hundreds of peasants even aid in targeting by making "close enough" still pretty deadly to the pesants.
Meanwhile the Dragon is within reach of most arrows and many spells.
If the Dragon is too high an altitude, the boulder is easier to see coming and to be avoided.