Heck, sales and income taxes contribute to this. Why is Home Depot so busy on weekends with DIY-ers? Shouldn't an amateur's effective "wage" for finishing his basement be incredibly low compared to a professional with knowledge/tools/etc.? So low that he/she would gladly work a few more hours at the office rather than spending his weekend figuring out how to lay tile? The problem is that I am not taxed on my "earnings" from the weekend work I might perform around the house, but a professional is (unless paid under the table, of course). So, providing labor to myself is effectively subsidized. Furthering this is my employer's subsidy of my weekend labor in the form of health insurance, disability insurance, etc. The professional (or his boss) would have to pay for Workman's Comp insurance (at a pretty high rate for construction labor), but my weekday employer would foot the bill -- just as if I had hurt myself while out walking around the block.

Now, take into account the US's common labor arrangements -- salaried and hourly employment (as opposed to piece work, commissions, etc.). Let's say you are paid a fixed salary. The benefit of spending a Saturday in the office doing extra work to impress the boss has an uncertain payout far in the future -- one which might rationally be discounted. However, the benefit of finishing one's own basement instead of shelling out an extra few thousand in labor is tangible and much more immediate. It is no wonder mid-level salaried employees show-up at Home Depot on weekends. The lot for hourly employees is worse. For some strange reason, the US requires that they be paid 1.5x/hour for work beyond a 40 hour week. 2x on Sundays. This is a huge dis-incentive for an employer to offer his existing employees the opportunity to work weekends when business is heavy. It might be possible for someone to pick-up extra, part-time work in areas where he has expertise, but this work often is hard to come by. So, Home Depot flourishes and basements are finished with off-kilter walls.

Personally, I have had multi-month periods as a software contractor where a client said, "work as many hours as you can, please!" So, every hour not working had a real, easily-quantified opportunity cost. I would have been crazy to have mowed my own lawn. It would have been nice to have been able to purchase other "services" like a doctor's visit without a wait. I even tried to offer the doctor's office a premium to be seen in a high-priority queue, but most insurance plans don't allow such upcharges on top of pre-negotiated rates. You can probably look at my HN posting history and figure out the periods when I had effectively unlimited earning potential -- there are huge gaps where I barely had anything to say.