Buying defensive early is a death sentance to your relevancy for the next 10 minutes of the game, that's part of the marksmen problem.
You cant efford to build anything but Botrk/storm razor or you will be so behind you will never come back.
Yes, maybe karthus ult wont finish you by it's own, but you still wont be able to fight him since his poke is constant so he has no problem popping it then going all in, but at the same time, you put yourself so behind in terms of damage, karthus will just roll over you with his AoE and slow and you wont be able to trade with him.
That's not even counting OTHER champions who arent karthus.
This is a DH and marksmen problem tho, karthus as a champion is fine