But we can take those official parts and get our friendly repair shop to do the hard work. This is great.
That's up to the shop. Most shops are going to turn away that kind of business (try bringing containers of motor oil and an oil filter to your local auto shop) - it's simply not as profitable as supplying (and marking-up) both parts and labor. Given a preference for what jobs to take on, they're not likely to accept a job that is substantially less profitable.
Now, I've done plenty of "skilled amateur" repairs for friends and family - typical home handyman stuff, as well as some computer repair. I'm not in it to make a profit, so whether someone hands me the parts or I buy them and get reimbursed, there's no mark-up on that cost.
I was also hungry enough, in my youth, to do freelance, labor-only repair/installation work in various recording studios - I charged for the time I spent on purchasing, but my client paid the suppliers directly. I didn't have the money to pay the suppliers in advance and then wait for my client to pay me at the end of the job. I also didn't do a sufficient volume in parts and electronics gear to qualify for a contractors discount.
But basically, once the enterprise scales up from "skilled individual working from home" to "friendly repair shop," the economics necessarily change. Rent, insurance, salaries, etc. take the "labor rate" paid from "100% profit" to a substantially lower percentage. Without the sale and markup of parts, the typical repair shop would quickly go out of business.