There's usually some overhead involved too: finding labour, scheduling appointments, bringing your car to the dealership etc. Doing your own oil change takes less time than driving to the garage.

You don't have to do everything on your own, but I'd save the experts for the important challenges.

Not to mention that a job that it might take a service 1h's worth of labour to do may take me 2-3h, amortized with the learning curve (say, 4-5h first time I try, and eventually down to 1.5-2h after I've done it a few times).

I do my own motorcycle repairs, because I sometimes stray far from the nearest mechanic, and field repairs can be necessary. I also cook a lot, because I have to eat every day and restaurants are expensive.

However, I paid someone to rebuild my forks, and I buy pre-made pie shells. Those things are not worth doing myself. My father rebuilds engines in his garage, but won't change the oil on a slush-covered car. He'll pay someone else to do it.

I guess it depends on how pleasant the job is to you, how often you need it done, and how much it costs to get it done relative to your own salary.

Yes, this is the most important point I think.

That cuts both ways though. I'd rather work 28 hours/week and spend more time fixing stuff myself. But jobs largely come as 40 hours/week (there are exceptions, but they would mean compromising on field, location or both, at least for me); if I could work 70% of the time for 50% of the pay I probably would.

The more I've read about the lives of 19th century pioneers the more I've realized that even they depended on something more akin to modern supply chains than on real self sufficiency.

Truly self reliant peoples live in poverty. I know no counter example.

You cannot help anyone else unless you have some minimal level of self-reliance. That’s why airlines tell you to fit your own oxygen mask before trying to help anyone else.