Here's why:
* Ahsoka leaves the Jedi Order in *Star Wars: The Clone Wars*: Ahsoka Tano, Anakin Skywalker's Padawan, leaves the Jedi Order in the final season of *Star Wars: The Clone Wars* before the events of *Revenge of the Sith*. She is falsely accused of a bombing and, despite being exonerated, chooses to leave the Jedi Order due to the disillusionment and distrust she experiences.
* Anakin's apprentice is never explicitly mentioned in *Revenge of the Sith*: Anakin is shown to be a Jedi Knight without a Padawan in *Revenge of the Sith*. The film focuses on his internal struggles and descent to the dark side, rather than his role as a teacher.
What happens to Ahsoka after leaving the Order?
* She becomes a vigilante: After leaving the Order, Ahsoka becomes a vigilante, protecting the innocent and helping those in need.
* She reunites with Anakin during the Siege of Mandalore: In the final season of *The Clone Wars* (which takes place shortly before *Revenge of the Sith*), Ahsoka and Anakin reunite during the Siege of Mandalore. This is their last interaction before the events of *Revenge of the Sith*.
* She survives Order 66: Ahsoka is not shown to have been killed during Order 66, the Emperor's order to eliminate all Jedi. She survives the purge and goes into hiding.
* She continues to fight for good: Ahsoka continues to fight for good, eventually playing a role in the rebellion against the Empire in *Star Wars Rebels*. She then appears in the live-action series *The Mandalorian* and *The Book of Boba Fett* and is confirmed to be a key character in the upcoming series *Ahsoka*.
In summary: Ahsoka Tano is not Anakin's apprentice in *Revenge of the Sith* because she left the Jedi Order in *The Clone Wars*. After leaving the Jedi Order, she becomes a vigilante and survives Order 66. She later plays a significant role in the rebellion against the Empire and continues to fight for good.