Regional Attrition
With the industry stagnation, many have stayed where they are. I know several who got stuck at the regionals and became CAs and are making good money and have 3-4 weeks vacation and good seniority. It can be very difficult for some to go to a legacy or a major carrier to start all over again and lose that seniority, accrued vacation and take a huge payout and hit in the QOL.
Yeah, you'll say that you'd have to plan for it and save up. I agree. But realistically, the percentage of those pilots never have.
Also, I don't care what anyone says, age can be a huge factor. The legacies and majors would hire a 35 year old much quicker than a late 40s something in a minute. I've seen and experienced this. But, at this point, for me to move on and take a pay hit, would be very hard to recoup. If you're young or have a significant other with a good job, it won't be as painful.
Luckily, I have a 4 year degree in a field outside of aviation which I can fall back on if needed.