Amanda Bynes chose to leave "All That" to pursue other acting opportunities and her burgeoning film career.
She had already begun to gain recognition in movies like "She's the Man" (2006) and "What I Like About You" (2002).
It's important to note that Amanda Bynes did not give a specific reason for leaving the show publicly. However, the timing of her departure coincides with her growing popularity and the desire to explore different roles.
It's also worth noting that "All That" was coming to an end in 2002, so her decision to leave may have been influenced by the show's ending as well.