Gameplay wise, it's because humping around thousands of arrows that cost 100k per stack has been proven to be absolutely atrocious. Rangers in XI had to buy arrows by the truckload to do anything, and if they ran out, they may as well just kill themselves and go back to town because their usefulness immediately ended.

Lore wise, I guess you could abstract the repair cost as buying all the arrows you'll need for the day. Maybe people just send you arrows because you're the Eorzean equivalent of Kanye, so every time you wake up your retainers just dump a big barrel of arrows in your inn room.