The thing is its 160 at max rank. Most ultis at that rank are between 60 and 90. If we take the average death timer for those levels is around 1minute that means that most champions will have their ultimates available by the time they reach a fight situation. Karthus will have still over a minute left on his ulti CD though. You can force fights at turrets or baron knowing that the enemy will be missing out on at least 600+damage per champion. I also believe that if Karthus ultis after his death (highly likely to avoid an interrupt) he wont gain the CDR from blue buff for it, I cant think of any CDR items that Karthus would build so he isnt really reducing its CD the same way other mages are.
When you take into account how squishy he is and his lack of CC he is a perfect dive target if a team tries to defend a target, especially with the short range on his AoE clear meaning, he isnt Anivia that can just put her ulti up and walk away from the turret to stay safe, I dont think there are many other people outside of mages that are particularly good at wave clearing safely either, so an inclusion of Karthus in your team would mean you are taking that risk.
The reward to playing Karthus is meant to be the increased chance of kills that may have escaped your team, as is quite obvious from his complete lack of play in competitive tournaments his reward right now isnt outweighing his downsides. I think its almost safe to say I've not seen a Karthus team win a competitive game since his nerfs.