Expendables such as ammo, especially when this ammo is required to make a job stay on par with other jobs, should be easily craftable, or a slightly worse alternative should probably be obtainable from an NPC shop. Same reason why ninja's throwing shit never took off. However, nin always had melee to fall back on, and they were also good at tanking stuff so they always had a good role to play. Ranger is however a ranged job, shooting stuff from a range is their game, and for that you need a steady supply of cost efficient ammo.