Technology jobs are very highly specialised. And I'm not just speaking of computer tech.
I'm old enough to have seen several generations of obsoleted "gold tickets". Nuclear Engineering, future destroyed March 28, 1979. Freeway engineers, the freeway protests of the 1960s/1970s. Big Three auto manufacturers, dream destroyed with the first oil embargo, and sudden onslaught of more fuel efficient, less expensive, and better quality imports from Japan and Germany. Petroleum engineers, dream destroyed as the oil market collapsed in the 1980s. Aerospace engineers, dream destroyed with the fall of the Soviet Union and redundancies in the military-industrial complex throughout NATO powers.
Travel agents, newspaper journalists, and Perl hackers are only more recent victims of the same levels of specialisation.
It's not that these people are dumb, many were brilliant. But they're also laden with a tremendous amount of arcane knowledge, that, unfortunately, gets in the way of learning new skills.
It's been painfully obvious for a long time that, individual success stories notwithstanding, retraining and "reskilling" typically does very little to alleviate this.
Yes, future generations get trained into new skills, some of which may still be germaine 4-8 years after they've completed schooling. But my observation is that job and skill cycles are iterating faster.
So that's a part of it.
Yes, there are other dimensions. Obligations, family ties, social ties, fears, the fact that the most mobile do jump ship first, and more.
But really, it's quite crushing. You may not understand it now, but you almost certainly will, eventually.