Why do people keep saying karthus is a noob champ?

So whenever I play karthus (Which is a lot cause I can carry really well as him now) people always say im a noob champ or "its so easy to press r" and I am like, Karthus is a hard champion to play, and I always poke so hard in lane, and in team fights I try to be ranged, and if it looks grim, die then ulti. So why do people always get mad at me when I play karthus?
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