But low-pay + labor shortage is seems to be a common thing.
I would speculate it's driven by the production of a very price sensitive commodity. Lots of people choose the cheapest flights possible regardless of other considerations so a significant rise in pilot pay might make a given airline unprofitable since they can't raise prices to compensate.
So airlines feel strong pressure to keep wages down.
And really, you could say a "labor shortage" is what every high volume, low margin operation complains about but the reality is basically "we pay low wages so it's hard to get enough people to work for our low wages". (Obviously, with airline pilots, it's low wage relative to the cost of gaining the skills etc). We can see similar language with H1b visa proponents and even fast food restaurant operators use it periodically.