Because they are intended features. Things like animation canceling, resets, and such are thing that increase the depth and richness of the game through its mechanics and allow more avenues players to grow and improve through their time playing. They are things that are not connected to Kalista being specifically unable to stop basic attacks that have been declared, whether the basic attack was intended by the player or by accident, and it does not make sense to arbitrarily remove mechanics from all the champions because Kalista cannot cancel one specific part of one of her animations that is the result of how Riot chose to implement her ridiculous passive. This game already has a problem with draining the pool.
So because:
It would suck to die because you are stuck in place because you have autoattack turned on and can't cancel the animation. Especially when your champion is not the one who has a dash on every attack.
Kalista can cancel animations.
It would remove intended ability and strength from the champions.
It would remove depth from the gameplay.