I can think of a lot of things that most people would be better off doing themselves, but it doesn't surprise me, since we're biased to think anything we haven't done before is really hard.
In terms of unsurprising things that people should DIY:
Just because something is an easy thing doesn't mean you will know it's an easy thing. When you figure out whether it's worth it you need to consider the chance that it may look easy but is not really easy. After all, if you don't know anything about insulation blowing, how would you know if there's some way it could go badly wrong that you haven't heard about? Also, you're ignoring the cost of getting the knowledge to do the easy thing. (Really, I'm supposed to know I have to go to a tool rental store?
Just because something is an easy thing doesn't mean you will know it's an easy thing.
That's pretty much why I made this thread: so that I (and others) could learn that something that we didn't realize are easy actually are.
We have the internet now. You can look up how to do these things.
And regarding renting from the hardware store: it seems to work out fine for most people, but I got a blower that didn't work right (since hardware stores don't maintain rarely used tools well) and that made it take a lot longer than it should have (mostly driving because I don't live anywhere near a hardware store or tool rental store). The worst case scenario if you make this mistake is that you return your (free) hardware store rental and go to a tool rental store anyway. I'm just trying to save other people time if they try this.
We have the internet now. You can look up how to do these things.
But where do you get the knowledge to know that you picked the right guide off the Internet and that it isn't going to violate housing codes that don't exist in the area of the person who made the guide? Or how do you know that it isn't going to have a long term chemical reaction with the floorboards because the guy writing the guide didn't have such floorboards and didn't bother to mention the possibility? Or any of many things that could go wrong? You need knowledge in the first place in order to know which source has knowledge you can trust.
And regarding renting from the hardware store: it seems to work out fine for most people
I didn't even know that there is such a thing as renting from the hardware store.
I think you're being too risk averse here. How do you know you hired a competent person? Can you really be sure that they didn't do something stupid? Are you sure you won't need the money you spend on them for something else? Obviously you should do a cost benefit analysis, but in the cases I mentioned, the costs are way too high for basically no benefit besides saving a small amount of time.
You know that you hired a competent person because they
Edited to: I am not surprised that people pay for: vetting cats, repairing shoes, adjusting clothes to fit, (most of) hairdressing, repairing more difficult issues with the bathroom equipment, & sometimes, food.
Actually, I try not to be surprised about people paying for something instead of doing it themselves, because it reminds me of the fundamental attribution error and I just tell myself they have their reasons, or are expected to have their reasons (like having their wedding photoes done by professional photographers in specific locations and poses.)
To what question are you responding? If we go by the title, you are surprised by people who pay for vetting cats?
I misunderstood the title. Will edit my comment.
(Although I was surprised when we failed to remove sutures from our cats' stomaches; I really expected it within our capabilities.)
Two of the main resources people have are time and money. The world offers many opportunities to trade one for the other, at widely varying rates.
Where do you see people trading money for time at unfavorable rates - spending too much money to save too little time? What things should people just DIY?
See also the flip-side of this post, "what are you surprised people don't just buy?"