If you can make $100/hr and can pay a contractor $30/hr to do something, why should you do it yourself (especially if you work full time and would rather use your free time for something else)?

If the skill is quick to learn, does it make sense to pre-learn it (and maintain the skill!), or just have the resources on hand to learn it if necessary?

I think this is presupposing that your skills are fixed and if anything happens you're stuck with always being dependent on others even if it's a really bad deal for you. That's not the case for most of these

Finding someone who will take your money is easy. Finding that they can actually be trusted to do the work correctly, on budget and on a compatible schedule may or may not take more time than actually doing the thing itself.

Also, if you do not bother to learn at all, you have no way to verify that the work is actually done correctly. People will absolutely take advantage of your ignorance.

A lot of these skills generalize way beyond the immediate problem. I don't have a "fix the faucet" skill, I don't even think in these terms - more in terms of "operate wrench", "operate set of screwdrivers", "understand how water faucet works", "understand how plumbing works" (at high level), and these skills let me diagnose and fix many problems myself, most of them not even related to water faucet. Etc.

Other considerations:

- Sometimes you need a solution NOW. Household infra tends to break down on Saturdays and Sundays a lot for some reason.

- Sometimes it's not even about efficiency. Like, we got ourselves an allotment garden recently. Per your $100/hr vs. $30/hr math, I could and should just hire a bunch of contractors to clean it up and set it up for us. But for me, this defeats the point of getting that garden in the first place.

- Some work is too small for a contractor with necessary skills to come, unless you let them bill you for 10x the work. I had to learn how to patch holes in ceiling myself, because the price for getting the 4m x 5cm hole fixed was the same as for renovating the whole ceiling.

EDIT:

Also what 'lotsofpulp said - sometimes you want the outcome to have a very particular shape or quality level, and to guarantee it, you'd need to micromanage the contractor so badly, that in many cases it's easier to do it yourself.

There is a lot of your time involved in hiring someone else even if not active labor. And potentially stress in finding a contractor who will do a good job and the right job.

Doing it yourself can be rewarding and potentially much quicker. Why not do it yourself if you're able ( and assuming its not one of those things a pro could do in 1/10th the time).

You have to make the extra $100 per hour to pay the $30. So the employer has to agree to hire you for more hours. Don't forget you have to find unpaid time to find a person who would do the job.

It depends on the volatility of outcomes of paying $30 per hour for someone else to do something.

I like to do things myself or learn how to when I cannot sufficiently predict the quality of the service I will get or if the damage can be too great.