Yeah, I have no intention of moving left until I could hold a decent line. Our reserve CAs get beat down hard.
Other potential, but less common reasons, are medical (can fly with second class in the FO seat here, don't need first class), and there are a few folks who failed their first try at upgrading, and probably aren't willing to risk a second failure, which would generally mean their termination as an employee.
The left seat is as hard as you make it… if you’re someone who can do your job and make decisions its not much harder than being an FO (just know where to look stuff up in the FM/FOM). There are plenty of CAs who go out of their way to do lots of other peoples jobs and solve other peoples problem and make the job so much more stressful than it needs to be.
Most people who fail upgrade can’t make decisions (either appropriately or timely). This not usually a problem for someone who has been an aircraft commander in the military.