You had to start with whatever small regional airline or cargo company you could get to accumulate hours, and eventually you could get into the airlines. Until the last 10 years or so, there wasn't really a shortage of pilots in the airlines.
Plus, as we know, non-executive salaries haven't been keeping pace with inflation. And perks and retirement benefits have been shrinking. So it's less attractive now to become a commercial pilot than it was 20+ years ago.
A couple of other factors are also involved. 1. The US economy has shifted away from trade/skilled jobs and into information jobs. 2. Many pilots of the past learned to fly in the military; but since WW2, we've had less and less need for pilots - especially now that we are moving a lot of activity to remotely piloted drones.
I expect that as commercial pilot jobs become more attractive (due to not enough applicants), the salaries may go up, and the interest will redevelop.