Because of the sheer amount of power it has regardless of whether or not it's used.
There is no other ability that everyone uses whatever protection they have against it every single time, with the only exceptions being when they're about to back and mostly full health or high health and in base. If Karthus is ulting you, you use that Zhonya's, Stopwatch, untargetability, Edge of Night, or whatever other protection you have. If he has it and is alive, most people won't start fights. It makes all trades significantly more risky as soon as he hits 6, because, yes, you can survive literally anything your opponent throws at you, but Karthus can finish you off from literally anywhere. It is the ability that single-handedly creates the most map pressure out of any ability in the game, with the only other ability who has any potential to have more being Shen's ult, and that only does so situationally.
Karthus's R is highly telegraphed. It's a channel broken by any CC. It can be evaded by a huge number of things. Yet it still has absolutely massive pressure by virtue of being up. By pressing R he can either force you to blow vital cooldowns or lose a significant portion of your health. Yeah, Fizz can just troll pole it, but then Fizz has made himself temporarily vulnerable to any gank or trade. You can just use Zhonya's, but now you no longer have it if your opponent decides to attack you...and Karthus can get his R down below 120s with just normal 40% CDR and level 11, so he'll have it back before your next one. And after a 3v3, 4v3, 4v4, 5v3, or 5v4 in jungle, when you've blown your cooldowns...Karthus can ult you to clean up perfectly safely.
His ult's power resides in its sheer pressure, which is why it has such a long cooldown, because otherwise the pressure is just too constant.
Cutting its cd in half would result in being able to force stasis, invulnerability, untargetability, and spellshields out of an entire team every 80 seconds in lane, with just 20% CDR, and Karthus always has Presence of Mind or Ultimate Hunter, if not both. Late game, it's 48 seconds with just 40% CDR. It'd just be too much power up too often.