This mechanic balances long range with long cooldown:
The rule is :a(high-end, high-cost typically) spell cooldown ends when the spell hits its target(cooldown lasts as long as projectile flies).
I.e. short range attack ends fast, and cooldown is short. Long range attack projectile cooldown is slower, due to having to travel longer distance.
This has to be balanced with +faster projectile speed modifiers(from skills/items) so overpowered shot-gun spells don't exist(projectile need to be slow enough to be dodged/avoided manually).
Rapid-fire spells/skills such as arrows, should use range damage decay instead, as on-hit cooldown is meant to discourage long-range use.