The simple fact is that there are just not enough reasons that makes one want to be an airline pilot.
Some of the downsides are:
Expensive outlay in initial training through to Commercial Pilot Licence level.
Huge time investment in hour building after that, flying usually as an instructor, hacking about with students doing their best to kill you, for nowhere near enough money to live on without a second job or two.
Even more expense to get multi engine rating, turbine rating...
Then you get to sit your ATPL.
Then if you're lucky you might get offered a job as first officer (copilot)
Then you have to do a rating on the aircraft you're going to be flying, which you'll have to pay for, and is generally stupendously expensive, or your employer pays for your rating but you are then indentured to the employer for years. All the time earning diddly-squat.
Ascending to captains chair, or onto larger types, is usually seniority based, and if you want to move to a new employer, you go back to the bottom of the ladder.
Most of the upsides are:
You get to fly planes for a living.
You get to wear a pilot hat and put bars on your sleeves.
It's just not an attractive job any more. It's not even an "impressive" job any more, once upon a time, pilots were seen as near enough to gods, today, they are barely a step above your local bus driver.
For some, getting to fly panes for a living is enough,they just love flying *that much*. But there are not enough of those people to meet the demand.