Why does WWE do this to people?

WWE has done this a lot. People like Mark Henry, Kane, Vladamir Kozlov, and Khali come in as unstoppable monsters. A good example is Sheamus, Cena had faced everybody and they needed to give him a challenge and Sheamus won the WWE Championship. Now Sheamus is a mid carder. Even if Khali stayed being a main eventer the IWC would complain about him being overrated.