The main reason msot people aren't playing karthus (take it from an Karthus main in Season 0 (best days of karthus life) is he just hard to master for most people, cause you can't leave your E on nearly 24/7 and it best to leave off early game as while it off you get mana back when you kill minions , but when you are vsing high mobility common pick for mid laners, like Yasuo , Zed and talon , it hard to pin them down as they have ways to avoid the Q and then you burn lots of mana with E active.
Also not many people know when your undead Karthus you can cast Q/Q/W/R where they are jsut spaming Q's only, and most the champion are being play are higih moblity right now, and also ways to remove the slows/debuffs.
and Also cause Soraka is an high pick rate with people it kinda counter karthus ult, since she heal more then they are below x% hp.
but i do get to play my Karthus from time, to time.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2150560553/43676701?tab=overview
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2156163805/43676701?tab=overview
So i can understand why you feel that, but just know usely you wantto build around your team , so if your team lack the AP and more AD , then depend onthe ap you need, if you can just be an team fight ap , karthus is an good pick, but if it lane control, and ect, you need other ones , that are more viable .