Final answer:
Mazer Rackham is still alive in Ender's Game because his death was a simulation. He was kept alive to train and mentor future commanders.
Step-by-step explanation:
In Ender's Game by Orson Scott Card, Mazer Rackham is still alive because he was not actually killed in the earlier battles. Instead, it was a simulation created by the International Fleet to test Ender's abilities. Mazer Rackham was kept alive for decades in order to train and mentor future commanders, like Ender, on how to defeat the Formics.
Learn more about Ender's Game