"karthus is a late game"

Late game? Don't you know that is the domain of the marksman?

Silly mage.

Eh. He IS a late game. But Karthus is an unreliable character. By the time it gets to that point, many characters have their Movement Speed items for their respective classes, and while it would be NICE for, say, a Marksman to hold still and take the Single Target Q "Liek a MAAAAAN!", chances are they won't.

By late game that's when everyone has their Guardians, Zhonya's, Aegis, etc