What a lot of people don't consider is all of the things Steve listed...plus there's no guarantee the mechanic is working 40 hours a week. Especially if he is running the business by himself. I charge quite a bit... but then several hours a week I am the accountant... repair guy.. floor sweeper... ordering equipment and shipping merch back to clients.. blah blah blah.
If I got my hourly rate @40+ per week... yeah, babay.. then I would be rolling in some dough.
AND.. on the weeks I do have 40 hours of work... all of that other crap still has to get done... so it's a 60 hour week... which diminishes what appears to be a high hourly rate.