I feel like you're making a pretty big assumption here: that people care or want to think about the details of these types of jobs at all, let alone do anything

I feel like you're making a pretty big assumption here: that people care or want to think about the details of these types of jobs at all, let alone do anything themselves. Part of the reason why a plumber or mechanic is so expensive is that you're buying not only the solution to the problem, but also your own time and energy that you would have spent thinking about and implementing that solution.