A few thoughts from an accounting perspective:
Opportunity costs (the other things one could have done with hours spent on something) are not recognized costs. They most certainly should be part of the decision making/planning, but they don't get recorded as costs.
Some people, myself included, are prohibited by employment contracts or laws from engaging in the practice of their profession other than for their employer. As an overtime ineligible manager, my salary is fixed and cannot be increased by working additional hours. I cannot get a second job or moonlight in my profession. I could flip burgers or don a red or orange apron, as there would be no conflict of interest. Then I would pay taxes of 40% on my $10 or so per hour, before turning it over to a contractor. So, I'd be working 10 hrs or so in one of the boxes to pay for an hour or so of pro time. The fact is, my day job is 100% mind work; it's a welcome break to do things with my hands.
Overall, taxes create a tremendous, if often overlooked, incentive to be self sufficient. If Al gets paid for painting a room in Bob's house, and Bob gets paid to mow Al's lawn, easily half the money exchanged could (or, rather, tax codes say should) go for taxes, at least in NYS. (8% sales tax; 15% federal self employment tax; 7% state income tax; 25% federal income tax; some added on to payment; some deductible when computing others). So, even if each gets paid the same, one has to work 2 hours to be able to purchase one hour of the other's time.