Why can you cast spells while stunned?

Every champion in the game, currently, can cast a spell while stunned.

If you are Garen, casting your E, then if you are stunned a second later, it still goes through. If you are Jax, casting E, and then you are stunned, it still goes through. If you are Trynd and you cast E, then stunned, it still goes through, rather than being stunned on the spot. If you are Illaoi, and you cast Q and E before stun, it still goes through once you are stunned Aatrox, cast his Q before stun, and it still goes through. Talon, Ekko, Leona, Braum - the champion list is endless.

Why is this a concept that exists in this game? If you are stunned while your spell is in animation, that spell SHOULD stop instantly after being stunned, and should not apply any damage at all, let alone, being able to CC and stun people (such as Jax's E still going off).

What are everyone's thoughts on this? I think it's pretty stupid where you can still cast spells if you cast them just before you are stunned, they still go through.