Yoshi-P said the reason why arc doesn't have a ranged auto-attack is because you might use arrows when you didn't want to. Since they cost money, having them shoot when you didn't want to means you might waste money.

For this reason I also agree that arrows need to be changed in one of two ways:

1. Make all types of arrows available for sale from an NPC for really cheap.
or
2. Just make arrows be an item you equip that boosts your stats, but you have infinite of them when you equip it.

Then we can have ranged auto attack, because arrows won't be an issue of wasting money any more.