Why Karthus should be able to ult on "Untargetable" enemies

I was thinking a while ago, and I'd like to discuss this with the rest of the boards.

I'm a Karthus main, and I've noticed that a few of the posts about karthus's ult involve doing either %Health damage, or ignoring Untargetability, Invulnerability, or both. Personally, I'd think he should only get to do dmg to the Untargetable enemies, and I have a logical reasoning for this.

to help illustrate my reasoning, I'm going to use the Trading card game of Magic: The Gathering to help explain.

There are 3 particular mechanics in MTG that can protect a creature or make it "Untargetable" by spells and other abilities. And they are:

Shroud (Cannot be targeted by ANY spells and abilities) Hexproof (Cannot be targeted by spells and abilities your opponents control) Protection (Cannot be targeted by anything from a particular source, like Color, Creature type, Card type) Example: Protection from Blue, Protection from Creatures, and Protection from Instants

However, these do NOT help them from any card that affects the entire BOARD like the card "Wrath of God", which is a sorcery that costs 2 colorless mana and 2 white mana, and destroys all creatures on the board. All creatures will be destroyed despite whether or not they have the above 3 mechanics BECAUSE OF THIS REASON:

While the above Protection mechanics prevent the creature to be targeted by spells and abilities, Wrath of God doesn't follow the Target rule. Instead it follows the ALL rule, which goes around the Untargetability clause of Hexproof, Shroud, and Protection.

Using this knowledge, we can use this to apply logic between Karthus's Ult and anything that causes the user to be untargetable.

First off, let's look at Karthus's Ult:

R - Requiem: Karthus channels for 3 seconds and, upon completing the channel, deals magic damage to all enemy champions.

Karthus cannot move or deactivate after activating Requiem until channel ends or is interrupted.

I purposefully put All in Bold and Italics because of what I'm trying to get at here. A champ like Fizz with an ability that makes him untargetable means he cannot be TARGETED by Champion Abilities right? Well, I think Karthus's ult should follow the rules of Wrath of God and overrule the Untargetability clause

Also, I'd like to clarify that Untargetability and Invulnerability are 2 different things, Invulnerability is the inability to die due to damage to destruction, similar to the MTG Mechanic Indestructibility. Cards like Wrath of God cannot destroy cards that are Indestructible, and Karthus's R wouldn't work on champs that can make them Invulnerable (Kayle Ult, Zhonya's,Kindred ult, Bard Ult, etc)