Mostly QoL of being a captain in base at a major airline already. A huge majority of CAs I've ever flown with at F9 have no intention to leave, nor have their apps out. Many of them stopped applying when they got the CA seat in-base. Some can't afford it (literally), some don't want to take the paycut, many think they're "too old" despite only being 38-44 years in age.. The list goes on. Each have their own reasons.

The captains I tend to see leaving are the brand new, <3-3.5 years on property CAs who are still 28-33 years in age, or the very senior that are just fed up with the company and want to make the most of what's left ahead in their career.

FOs have nothing to lose - our year 3 pay is only ~1-3$ more per hour than a legacy's year 1 pay..and the difference is likely made up with the soft pay opportunities those airlines have. For many of us it's a no brainer to leave.