Why does Sol's W have a cooldown

Its called counterplay breh. L2Dodge dem cropdusters.

Because he was way, way too oppressive when the cooldown was shorter. Being able to damage people without having to stop moving is an absolutely huge advantage.

That would be pretty OP for laning or something.

A better question would be why does his stars contract to disappear when he's CC'd. It looks dumb and honestly if you get hit by those things when he is immobilized that is your own dang fault imo.

I understand that it would have a cooldown early in the game, since that's when ASol is laning against his opponent and should not have access to unlimited poke, but later on I think it would make more sense for him to be able to extend and retract his stars at will, since then it becomes much more about gauging the distance between himself and his opponent, and less about hiding behind a mostly-static minion wave. In fact, it might be better to also drop the 50% damage increase and keep the damage constant across distances, while also adjusting his star damage accordingly to avoid sinking his DPS. Celestial Expansion is already advantageous early on for the poke it gives, but later on Aurelion Sol should choose his stars' distance based on how close his target is to him, not on how much damage one mode deals over the other.