Viegar warning before stun why is even there?

Rupture

(Q) Lay Waste

Most AOE stuns in the game have some sort of warning or travel time to make it so people can dodge it if they pay attention.

Veigar's stun didn't have any sort of counter play to it so they added some and are tuning it accordingly.

I'm sure that soon they will do the same thing to other AOE insta-stuns, like

i mean no other champs that i can think of have this.

pretty much every champ gives a fair warning they're blasting CC in your face.

The vast majority of CC in the game is a skillshot with a projectile and projectile speed. Even powerful CC like Morganna's binding and Ahri's charm travels at the same speed as a brisk jog. And when it comes to point-click stuff, the casting animation and undodgable projectile from champions like Taric tends to take even longer, and have significantly less range. Kennen and Annie need to charge up abilities to CC things, showing a very visbale buff/debuff on themselves or their opponents. A lot of melee champs that apply CC are also, well, melee - which gives the warning of them essentially being a slower-moving projectile than most actual projectiles.

Veigar's capacity to insta-stun opponents at more than 1000 range away? Extremely unusual, especially for a QWE ability.