Historical attrition rates at one of the big three was running about 100 pilots per year in addition to normal attrition (deaths, disability, retirements) which equated to about 35% of the pilot force. Thus every three years....a complete turnover by numbers. That outfit is estimated to be running 30-40 pilots short of needs daily thus requiring significant workover by pilots on their week off.
The costs of recruiting and initial training must be huge with that amount of turnover.