This is one reason why some still quit even after the TA was approved. Some companies just treat their employees better, simple as that. It isn't all about the money. Spirit payscale actually measures up reasonably well against other 737/A320 payscales if you're factoring in a 5 yr upgrade vs. a 10 yr upgrade elsewhere. Money being nearly equal (especially if going from year 4+ at spirit back down to year 1 elsewhere), those other considerations come into play.

Taking a paycut to go from flying the bus to flying a guppy, giving up a possible 5 yr upgrade for a 10 yr upgrade? Yea, that's a thing because you can tell if one company looks out for its employees and the other one doesn't.

Fun fact - that other airline with a similar payscale has apparently had only 2 training failures in the last 930ish new hires. That's a statement on the value they place on their employees from day 0. Interesting contrast in company priorities.