I don't know my self but I do agree with a couple possible reasons. However I usually compare what Arc/Brd are shooting to the bow used in that old Dungeons and Dragons tv series. Or in this case (since we also have a quiver) a bottomless quiver to draw arrows from. Personally I hated range jobs in XI due to the expense of buying ammo, and the RNG involved with leveling craft skills and harvesting resources on top of needing to have the highest tier rounds to use.. Either way lets be thankful we have a bottomless amount of ammo on Brd and Nin. Other wise they would consume way more inventory than anything else, and would always run out of them given how fast we shoot and how many can be shot on quick nock, rain of death and volley. On that note, lets also be thankful that Nin also doesn't need to carry sacks of tools to perform ninjutsu (thank you Naruto) as well as ranged shuriken to throw.