The description of the Zhonya's item isn't "Makes invulnerable so you don't die", it's simply "Put yourself in a stasis". So by having the function of automatically triggering the item to activate when you're close to dying contradicts the description of what it's supposed to do. Sure, it's commonly used to keep you alive, but that's not the only thing you can do with it. There are times when you can use it at full health or any situation, to juke or outplay. There are times when, even if you're low and about to die, you wouldn't want to use it anyway. Just like how people don't burn a flash if they know they are 100% dead, it's a waste of a cooldown when you know there was no point in using it. You basically make it so that the enemy can force you to burn your resources when you don't want them to, giving you a disadvantage. That's why they're active items, you use them when you deem they are necessary, not when the game predicts it would be helpful.