Why aren't finishing moves a permanent thing?

The URF icons that allowed you to either call down a laser beam, make a love heart explosion or summon urn's ghost when you kill someone, and now these all stars fire and ice ones. why cant we permanently personalise death finishers. i know that if things are too visually spectacular like the ice/laser/heart finishing moves that it can distract from or hide champions/abilities in team fights and thus affect gameplay, but smaller ones like the fire and urf ones shouldnt be a problem?
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