As of my last update in September 2021, Phil Mickelson joining LIV Golf, a proposed Saudi-backed golf league, was a significant development in the golfing world. However, the specific reasons behind his decision were not extensively covered in the public domain up to that point.
If there is any recent information or reasons for Phil Mickelson joining LIV Golf after September 2021, I cannot get this information.
But it is wiser to turn to modern and credible sources to find how and why Phil Mickelson decided to sign for LIV Golf. Information on why Mickelson decided to join the LIV Golf may be obtained from any golf news source, an interview with him or an official statement from the LIV Golf or the involved organizations.