In the decades after WWII, you got trained by the military and moved to the airlines when your time was up. As previously noted, the military has cut way back on flying and training pilots. Last Summer I bumped into a squadron full of C-17 pilots hiking Logan Pass - all in their 20s with about 200 hours. Bottom line the military can turn out highly capable complex jet pilots in under 200 hours.
Then there was the time when civilian flight training was affordable for the wages on offer to younger folk. Your first job would get you out of your parents' basement and pay for flying. You could then instruct until the next step on the ladder opened up. Much less of that now.
The shortage is of people ready and able to plunk down $100,000+ on a lottery ticket for a long internship in a crap job on starvation wages that may or may not lead to a half decent job.