Thus the starting out part. Certifications, years of experience, skills in terms of welding exhaust or the like all raise that, but mechanics get screwed on their rate regularly, which is why they're dropping out. See one of the other top level comments about "shortage -> low wages."
>breaking his back and knuckles, and exposed to carcinogens (used motor oil, grease, etc).
They work indoors and don't exactly work in an environment where things get burnt or aerosolized. It's no office job but it's not exactly ditch digging.
The Ford dealership I worked as a technician at had a covered roof, but just past the lift it was uncovered. It was basically outdoors - no AC and the wind would blow rain on you.
$20 is also on the low end for a good tech, you're correct.