Where have the pilots gone?

In 1987 there were 827,000 pilots in the U.S. Today, there are less than 600,000. Where did they go? How could they go when air travel has increased quite a bit? 

Part of the reason is that the Air Force in the 20th century trained about two-thirds of the pilots. This was before today's use of drones by the military. Now, most pilots have to pay for their training, which exceeds $100,000. The 1978 Airline Deregulation Act and 9/11 wreaked financial damage to major legacy airlines, five of whom declared bankruptcy.