The shop might be incompetent and strip the threads for example (very common) which will cost you a lot more money down the road when it's discovered and becomes a problem.
But, aside that, even competent shops can't provide the same quality of care than DIY because they have to operate on the booked time it takes to do a job. So they can't deviate from doing exactly what's on the work list.
Meanwhile, when I work on my cars I'm taking time to look around and pay attention and address any other upcoming problems I might notice even if they are entirely unrelated to the work I started out doing. And I clean the parts and nearby parts as I go, which a shop could never afford to do even if they cared.
Oil changes tend to be a loss leader so you can't often save money by DIY (but I always DIY due to the reasons above). But on other types of work you usually also save a lot of money by DIY.
For instance I replaced the fence between us and neighbor for ~$500 in materials. That neighbor had to also replace his other side fence (between him and third neighbor). Identical fence, but cost them over $4K to have it done by a contractor.
Because you like your car...
Every decade or so, I try a fast oil change place, usually when I'm in a hurry, and it sells me on changing my own oil for the next decade, because it's either obscenely overpriced, they screw up my car somehow, or both.
Last time I took vehicles in for oil changes, it cost nearly $200 for my truck (I know it takes a lot of oil, but ffs...), and for the car, they didn't get the airbox back together properly, so I had a nice post-air-filter leak. They even left the bolt they couldn't get lined up sitting in the windshield air inlet tray - because they'd not lined the filter box up right. And, they forgot a crush washer on the drain plug, so I had a dinner plate sized oil puddle under the car in the morning. Sorry, I see no point in paying someone to screw my vehicles up like that.
She called saying "they're saying 'the lug nut broke'"; I was thinking (and asking) "how is that possible (it can't possibly be a defective lug nut)?" 10 minutes later "the stud is broken"; 5 minutes later "they [Costco] admitted to breaking it".
The resolution was to have a local shop replace the lug stud and get reimbursed for the repair cost: $125 (the part might have cost $2)!
I chalk this episode up to them hiring warm bodies lacking any mechanical experience or aptitude (to the extent of not being aware of "righty tighty, lefty-loosey") and handing them an impact wrench.
But there's a reason that r/justrolledintotheshop talks about "uggaduggas" as a torque spec - to make fun of the people who just let an impact driver pound away at things like lug nuts. Cross threaded? Screw it, more uggaduggas! It'll snap when someone takes it off? Not My Problem!
Oddly enough, it's not always righty tighty. I rode motorcycles that had axles that were the opposite (so normal rotation would tighten them), and it was well known on the forums that if you let a shop just go at it, they'd try to loosen it, tighten it, smash the spacer, and your wheel bearings would then fail in short order. I always either watched the shop take my wheels off, or just took the wheels off myself and took them in to get the rubber replaced (alarmingly frequently - I was riding a lot).
So history argues in favor of it being their fault. In fairness, they were apologetic and never tried to absolve themselves of responsibility.
Regarding "Costco should have had zero problems replacing a broken stud on the spot", I wish it were so (would have saved a bit of hassle), but which expert person at Costco is going to be capable of doing this? They seemed to have shop nearby that they farmed such work out to, but this problem occurred on a Sunday...
That said... I'd have wanted to pop into the shop and personally inspect the remaining studs to make sure they weren't on the brink of failure as well.
It's also not uncommon for garages to under or over torque fasteners because they're in a hurry. My friend had his power steering pump come off the engine block while he was driving because the garage didn't bother torquing it properly after replacing it.
>Every decade or so, I try a fast oil change place, usually when I'm in a hurry, and it sells me on changing my own oil for the next decade, because it's either obscenely overpriced, they screw up my car somehow, or both.
The last time I resorted to having someone else change the oil in my car, because I was in a place where I couldn't DIY and didn't have time to go to a relative's place to do so, I used an actual garage and not a quick-change place, thinking they'd do a better job, I provided the oil, and they managed to overfill it by more than a quart.
The people that take jobs like these are simply incompetent; if they were smart, they'd be doing other jobs.
That said, depending on where you live and whether or not you have winter, any fasteners on the underside of your car may be rusted to the point where you need the oxy-acetylene torch you used in high school to get the damned things loose. The nuts and bolts on the steering and suspension systems live a particularly rough life.
I feel like it's partly a consequence of a highly specialized society. I'm not sure how to fix it. There's an opportunity cost. Why would I change the oil in my car when it's $40 to get it changed at the shop? Since I don't change my oil, I now don't know the basics of how to jack up the vehicle, I don't understand how to use the torque wrench to tighten the drain plug and I'm basically missing all the fundamental skills so I got no knowledge to build on.
The other contributor, is that growing up in the city, in an apartment, there was never any opportunity to work with my hands. I still recall a shop class in high school. I got to use a lathe and a oxy acetylene torch. That was many years ago now.
Now that I'm older, I make conscious effort to learn how to do things with my hands. There are things that I still must outsource, when they are on a critical path. For example if the car is down because the suspension needs to be replaced, I'm not going to spend 3 months researching and fixing it. We need it now.
I've come to really appreciate the famous Heinlein quote:
> A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyse a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
Im also very much attracted to the idea of a "renaissance man" [1] :
> Embodying a basic tenet of Renaissance humanism that humans are limitless in their capacity for development, the concept led to the notion that people should embrace all knowledge and develop their capacities as fully as possible.
"Specialization is for insects" indeed.
0: https://en.wikipedia.org/wiki/Competent_man
1: https://en.wikipedia.org/wiki/Polymath