Back in the 80's, when I first started wrenching on vehicles, the vehicles were much simpler to work on but you didn't have YouTube and forums like ours with a plethora of information at your disposal. You had to rely on what your buddy, dad or older brother knew or maybe you bought one of those shitty Chilton manuals.
Now, the vehicles are more complicated, but we have way more information due to the internet. If you are willing to take the time to learn and invest some money in tools, you can do a whole lot of repairs yourself. You probably won't be tearing apart engines and transmissions to rebuild them, but you can do quite a bit to save yourself some big coin.
There are legitimate reasons why people don't do their own work, like being super busy with career and family obligations or maybe you're disabled and can't physically do the work. But, if you have the time and aren't physically limited, it comes down to motivation whether you want to learn about auto mechanics. I guess I'm a sort of cheerleader when it comes to motivating people to do their own repair/mod work. You end up saving yourself lots of money while gaining knowledge and the satisfaction of doing your own work.
Ok, I'll step off the soapbox now.
Sometimes it's mind boggling how much information and know-how really exists on communities like T4R and Youtube. I wasn't wrenching in the 80s becaues I was a toddler, but I can only imagine it'd be way different from a confidence perspective if all you had was a damn manual (And a shitty one at that).
But, there is a catch to it all.
For as many people that exist who wrench on their vehicles even with the info out there, just as many or more exist that don't have the interest, desire, or time.
I think it comes down to those 3 factors with some exceptions.
Just take a personal poll of your friends/family, how many wrench and how many pay the dealer or a shop? My ratio is like 10 to 1 of friends/family that pay for stuff VS do their own stuff.
Honestly, I often pay for stuff I know how to do myself just because of work time and family/etc.
Gender plays in a role as well, this is one of the exceptions.
Example, I have two different female cousins, both who drive Benz's. Realistically, why would they ever wrench on them? Totally a 'guy' thing

-- and it's one thing if they have a husband or brother someone in their life who can wrench, but how many women dont? A LOT!
My female cousins with the Benz's also introduce exception #2 on top of exception #1 (gender) -- working on cars is often seen as a very blue collar thing, a lot of people simply have the $$$money$$$ to pay mechanics and shops for work.
A lot of my coworkers drive BMW's and I noticed they all take them to the dealer. As if the price of that vehicle isn't getting $hafted enough, lol. You generally find the German luxuries are harder to DIY on. A lot of special tools required.
I will agree with you that with the amount of info out there on Domestic and Asian imports, tearing into an engine or tranny is pretty much the only job you CANT DIY these days.