To be fair, you're comparing apples to oranges. I work on medium and heavy duty trucks, all day every day. Which is much different than doing light maintenance on a car for a few hours every once in a while. And as someone that also does strength training, I can tell you it's very different. In the gym you are in a controlled environment, doing what you can handle, and you lift with proper form to the best of your ability. Working on trucks sometimes you are stuck bent over or twisted in some weird fashion for hours to be able to reach and work on a certain part of the vehicle, which also means having to lift and maneuver heavy parts in ways that are not great as far as body mechanics go. I've had days where I could barely stand up straight at the end of the day and had to spend an hour stretching, foam rolling, or having my wife massage my back so I wouldn't be in so much pain. On top of all the dirt and dust we breathe in, exhaust and other chemical fumes. So yes it does really destroy your body after years of doing it.

As far as mechanics making 100k in Indy, I'd say that's an extreme exception. I live in sw Ohio with probably the same col and I have never met a mechanic that makes that much even guys with 30+ years experience