4 years before Brian O'Connor's retirement in Furious 7, Fast and Furious already explained his reason for leaving Dom's crew. Paul Walker was tragically and untimely killed in a car accident in the middle of filming Furious 7, forcing the film to re-assess his story and find a way to write him out of the blockbuster franchise at the end of the film. Instead of mirroring the real-life tragedy, Universal opted to keep Brian alive. Instead, it was decided that he would retire, explaining the character's absence in future films. That remains to be the case in subsequent films, including Fast X.

https://screenrant.com/fast-furious-brian-retire-reason-kids/