What Kamelchaser said. The more quals the fella leaving has the more it costs to replace him. For example, a TRE leaves (stop cheering), they need to train a tri to be a TRE. A LTC to be a TRI. A line Capt to be an LTC. An FO to be a CAPT and then travel the world shaking trees until they find, interview, test and recruit a new FO, and then train him.
Not cheap, nor quick. Each year they sit around and guess however many ungrateful lazy pilots will leave, and throw this number into a EGIT excel programme to work out how many newbies they need to handle growth and leavers. You would think they would have had enough practice to get it right by now, but alas no. Hence Crap rosters and ****e leave. But at least they are consistent.