Why is Irelia like this? (Stun length)

I play a ranked game as Ashe ADC with my Bard support vs. Vayne and Thresh bot. So, after a few ganks from a fed Shaco, we got ahead and the whole team is ahead now. In top, our Riven is just tossing Irelia aside like trash and getting herself fed. After a while, the game is good until Irelia ALL OF A SUDDEN just becomes this complete beast no one can deal with and then the whole game goes south. In the end, we lost, the team somewhat fell apart, and at that point Irelia was carrying.

So, I'm here wondering to myself WHY is her stun on such a short cooldown when it stuns enemies for what feels like an eternity? Like, tbh, I can excuse the true damage even though that can sometimes get annoying and everything else but WHY is that stun so long? She just outduels anyone with that stun unless you're someone like Nasus that has 500+ stacks and does a huge chunk of her HP with 1 Q or something along those lines.