Can someone tell me why is invisibility on non assassin class a thing?

First off, Twitch. Late game no counterplay champion that can 100-0 any given champion ( tank or not ) in less than 3 seconds while being out of range of everyone due to his ult. Having a peeling support ( Lulu, Nami, Soraka, Janna ) will make him even more uncounterable. Second off, Kaisa. Having X amount of attack speed gives you invisibility. Huge burst with AD and AP damage with a 500-600 shield so she can't get blown up while having true sight on W ( in example Pyke's W doesn't camouflage him due to her passive stack, no matter he's way out of her vision ). Next up, Vayne. True damage for free with no mana cost, and an invisibility with bonus damage. If you don't one shot her before she even blinks you are most likely dead.

ADC's are champions that are supposed to be squishy with high damage but very fragile. I don't see a single healthy thing about giving ad carries an assassin thing in their kit. Giving them stuff like Invisibility just makes them ranged assassins making normal assassins ( Zed, Rengar, Kha Zix, Talon ) a joke. Jinx, Varus, Ashe, Mf, Draven, Tristana, Jhin, Ezreal etc don't have any kind of assassin type of kit nor any power that is meant to be as an assassin champion yet they do just fine without them. Jinx can increase her range with Q and keep you not near her with her W or E zoning you and forcing you to either all in or leave, still a squishy champion and can get killed easily. Ezreal is a poking adc with one escape power yet he can be killed and he can deal a lot of damage if used properly, still counterable. Draven a manly all in champion that has the good old kill or be killed kind of gameplay. And so on. So many ADC's work just fine without implementing assassin class into their kit. Twitch, Vayne, Kai'Sa shouldn't be any different. You're making an assassin class a joke if you keep this like it is right now. Every high elo adc ( considering he knows to play any of these 3 given champions ) will delete your adc in a matter of seconds.

TL:DR Change all invisibility in ADC class. They do NOT need them. Keep it for assassins only. It's meant to be for their class. You don't see a marksman in WoW or any other game of that type that goes invisible and suddenly show up harvesting your soul in a matter of a second. Marksman should remain fragile, high damage, but vulnerable to enemy assassins.