(At the same time, people DIYing tend to underestimate their real opportunity costs).
The best outcome is that you end up liking tinkering and have pride in your work. That's a very high discount to the real cost.
Pride in work is a big thing. As someone who works at a desk, some manual labor on the weekend is a nice change of pace. It's also not pointless exercise. And in my experience the best way to get something done right is to get free advice from the pros and do it yourself. Because the people the pros hire to do it won't care as much as you.
Having your car or house fixed isn't that easy. It's not like you just tell some person "fix this" in 5 minutes or less and walk away, then pay them when it's over. If it's a car repair, you have to transport your car to the repair place, then either sit there while they fix it or find some alternate transportation, then you have to come back after it's done. If it's a 1-hour job, how much time will you waste just traveling to and from the repair shop, plus potentially waiting around for it to be done? You probably could have done it yourself in less time. For a house repair, you don't have to travel there, but you probably want to stay at home while the work is being done unless the house is empty, or you really trust the contractors and sub-contractors. So in either scenario, there's a lot of your own time that's not being accounted for.
And this all assumes that the professional will actually do the job correctly, and you won't have to deal with issues from them doing it poorly. This could easily end up taking FAR more time than the original job.
So in my view, if you know how to do something yourself, it doesn't need a whole team of people to do, doesn't require expensive specialty equipment, and isn't highly dangerous, it's quite likely a better deal to just DIY.
Still yes, the biggest issue I have is most times I hire someone to do something, it often goes sideways and I have to get deeply involved anyway. This effect seems to be even worse for white collar work, where "professionals" tend to push cookie cutter solutions without much analysis.
I was raised to fix it rather than spend wherever possible. Picked up a ton of useful skills that have paid me back a ton of money when I measure up with friends of similar age who used shops, contractors, etc.
Now it costs me more personally! It sometimes is just more work and energy demand than I feel good contributing because it can then take away from other parts of my day or days.
That all said, sometimes the money just is not there when the demand was! And so I do the damn work anyway.
To me, having to deal with people I don't trust one iota is a huge mental exertion too, that more than compensates for any physical or mental exertion needed for just doing the damn thing myself. And that's on top of all the time/effort needed to research places, compare prices, travel time, waiting time, etc.
Sure, if I had Alfred, my trustworthy hyper-competent butler, to take care of more mundane things for me, I'd let him do a lot of this stuff instead. But I don't, and the people that work these jobs (at least in the US) are usually anything but competent.
You could pay 100+ for a plumber to snake a drain or just buy a snake yourself and do it for <50 in parts and the next time it's free.
Perhaps for some tasks you buy so many tools it's the same price but the second you need to use them again it's a steep discount. Or you could ask a neighbor.
I've saved about $162/hour on various home projects when compared to quotes I've been given. Considering I'm a relative noob when it comes to this stuff I've gotta imagine they're charging much higher hourly rates than this.
This $162/hour is way more than what my salary as an hourly wage would be and it's also tax free to boot.