Karthus needs some serious buffs to his survivability.

He's a skeleton, he's always dead!

Don't know if joke.

Karthus has one of the strongest level 1s out there. Beyond that, he scales better than nearly any ap champion (only exceptions might be Orianna and Azir and Veigar), so there are compensations early game. In addition, getting 5 deaths on Karthus early isn't the end of the world because you only get stronger. Even when you die, you still get to ult and cast spells for free. They can't stop you, only run. Just a few observations.

Because a wall and his double damage q don't pressure and push people back enough