You are quite right; I came across a fair few "professional first officers" in my long career who had no wish to move seats. I think I am right in saying that all of them were bloody good operators and were good news to have in the other seat.
I think most of them had reached a happy level in their flying careers in which they were doing a good job but simply didn't want any more responsibility.
Interestingly, one of them had got shot down during WWII and had ended up in prison camp so maybe he felt that that particular command had not gone particularly well.
My favourite gave me a great bit of advice:
"Learn to live on a F/O's salary and that way you will never be in the sh*t when your company goes down the pan and you have to look for another job".
How true; how very, very true!