That’s because there is no single heavyweight champion these days. When I was a kid, there was one and only one champion (I still remember listening to the Patterson/Liston fight on the radio), and everyone knew who it was. Today, there’s the IDF champion, and the WBO champion, and the PTA champion for all I know or care.
If I were in charge, there would immediately be a tournament to decide who was the one and only champion at each weight class, and then that would be the only title.
Ring magazine lists Klitschko as the world champ and he is also the lineal champion.
Klitschko"s brother Vitali held the WBC title until he stepped down in 2013 to become mayor of Kiev. The two brothers vowed never to fight one another, hence the failure to unify the title. At one time, they held all the titles.