Usually it’s FOs with 1000 121 time at stagnant regionals whose pilot groups refuse to take a pay cut. There’s little movement and no new flying at their current airline and they can be an immediate captain at the top of the seniority list at brand new cut rate regional X (but for less pay) or spend another 5 years as an FO at their current regional, then a captain on reserve forever. It’s just capitalism and allows for lower airfare for customers, but it means we don’t get paid much.