We've known with almost certainty that there was a major impending pilot shortage since I started flying in 2008, but despite that the reward for a pilot who had just spent $100,000 out of pocket for training in 2008 was expected earnings of about $21,000 per year as a first year first officer for a regional airline. Sure, that pay scaled up quite a bit over the first few years, and there were well-paying and stable jobs available once they made it to a major airline, but despite knowing about the impending pilot shortage airlines spent the better part of the last decade scaring away all but the most dedicated applicants because the pay and working conditions were terrible. And now the United of today is paying for the short-term gains that the United of yesterday got from not bothering to nurture the pilot pipeline until the situation became too dire to ignore.

(There are certainly other factors too--among others the military's shift to drones has significantly slowed down that free training pipeline.)