Why does O'Neill leave Stargate SG-1?

Answer 1

The character Jack O'Neill was promoted to the Pentagon.

In real life, the actor Richard Dean Anderson wanted to cut back on the production schedule, and since he was one of the owners of the franchise, he got to do what he wanted.

Answer 2

Richard Dean Anderson, who played the character Jack O'Neill, decided to leave Stargate SG-1 to spend more time with his family and pursue other interests. His departure was also driven by a desire to explore new acting opportunities and to focus on his health and well-being.