https://www.looper.com/1914140/why-richard-dean-anderson-left-stargate-sg1/

Richard Dean Anderson's portrayal of Jack O'Neill was a primary factor in the success of "Stargate SG-1," so he had to have a good reason for exiting the show.

Why Richard Dean Anderson Was Missing in Stargate Season 6 (https://www.cbr.com/richard-dean-anderson-missing-stargate-season-6-explained/)

Richard Dean Anderson's Jack O'Neill was written out early in season six, leaving him absent and immobile for a few episodes.