I'd posit a potential reason that these fields are currently hiring is a combination of that it destroys your body without recourse and many of these positions require certifications that take a long time to achieve (either through apprenticeships or training programs). You will also generally not get any kind of meaningful benefits from these jobs, and your body will disintigreate before your very eyes as you work yourself to bone for a pittance. The compensation for these roles is poor in comparison to white collar work despite the perceived demand for them, there is no safety net in many cases (401k, pension, reasonable health insurance, etc. outside of union shops, which are rare outside of say welders and pipefitters (and getting rarer every day!)).

And frankly the work is miserable. I've crawled through suspended ductwork to run conduit and wiring in antifreeze recycling plants that were filled with god-knows-what reagents covering everything in dust thick enough to paint a clown. PPE be damned, my skin burned for days. It was hot, loud, cramped, wet with chemicals, uncomfortable, dangerous, and unpleasant. These jobsites are the bread and butter of blue collar anything; awful and dangerous conditions outside of your control, but required by your contract because not doing it means not getting paid.

Sure, an agent isn't going to be replacing the poor bastard who has to do that, but is our only response the the deliberate and systematic murder of the white collar job market "you can suffer for less money so you'll be fine"? That's a pathetic whimpering way to just accept the very loud and public murder of class mobility.