And sometimes there are very professional F/Os who simply do not want the hassle/responsibility of sitting in the left seat.
In my first airline after I left the RAF we had about half a dozen that I would call "Professional First Officers". One of them had been a Lancaster captain in WW II and he had been shot down and captured. He did not want to be a captain ever again and I can tell you that he was a joy to fly with.
Some of the others had worked their b*lls off and had reached the dizzy heights of having a CPL/IR and did not want to go any further. I think I can honestly say that they were also a joy to fly with.
Later in life, when our original company had gone bust and we were all flying for different companies, I got a call from one of them. His current company had gone down the pan and he was looking for a job.
I rang the Managing Director and told him that I had a great co-pilot looking for a job and that I knew him well but the big problem might be that he was 52 years old.
I shall never forget the response:
"I like my pilots to have no hair or grey hair".
Nowadays, he nor I, would have any chance of getting past the Human Remains department. More fool them.