As someone who got into academia "by chance" this was quite a shock to me.
Plenty going on in such a phenomenon I feel. There’s the relative directionless-ness that can plague smart people coming out of unchallenging high school environments; the often accompanying “prestige for the sake of prestige” logic that you can get in middle-upper class culture, and, IMO, the inflated value and prestige of actually having a PhD in the modern (ie production-line) education system. A healthy dose of cynicism about the corporate world is often involved too I’d say.
I personally find it unfortunate how classist and stupid Grad school would feel at times.
Of course, the level of competition changed dramatically between their generation and mine. Then you could go directly from grad school to a tenure-track position and generally expect lifetime employment. Now, in the sciences, you're expected to live a nomadic life as you go from postdoc to postdoc, making less than $50k, perhaps eventually landing that coveted tenure-track. In the humanities it's even worse, where you work adjunct jobs that pay less than minimum wage.
It's no surprise that people with family money are most likely to win these contests of endurance. If you need to earn your own money, it's simply irrational to persist in the academic rat race.
MD's kids are 24x more likely to become MDs as well.
They say declining pay and difficulties negotiating with bigger players such as governments and larger and larger healthcare employers in combination with the bad quality of life at work no longer make the costs of becoming a doctor worth it.