"My nephew got a iPad glass replacement by a “professional” repair company.. the glass shifts around (because they didn’t use enough glue) and the audio port no longer works. You get what you pay for. With Apple you get your device back in mint condition"
That kind of illustrates the point that you are arguing against - if they were easier to repair it would be easier to do a better job of it!
With regards to "you get what you pay for" - yes, you get pretty much the same quality parts without the apple stamp on them for about half the price. As an example Apple battery replacement in my wifes iPhone 6.. £75 cost for me to do it £25. Bought from a reputable supplier that I have been using for years - I have every confidence that this battery will last as long as the apple supplied one.. and if it doesnt I can still buy another two and not be out of pocket!