Heres another example of how people seem to be missing the point.
Bob walks into an Apple store and tells them his mid 2016 MacBook Pro needs a new logic board. Luckily they won't sell him one without having the serial number because otherwise he would likely have them order the wrong model.
They happily charge him $900 and tell him to come back tomorrow. If he brings back the old board, they'll give him $250 back.
He collects the part and takes it home where he should have prepared a clean, ESD safe work area with an ESD mat, ESD strap and the correct tools to hand. He should have this, but he almost certainly won't, so when he takes the new board out and handles it by the chips and connectors instead of the edges, and rests it on top of its ESD bag (the static lives on the outside), he has likely damaged the part and shortened its lifespan already.
He swaps the parts out but there is a good chance he will break a connector, bend a pin, loosen a chip or crack a solder joint, as well as deliver some more ESD from his shagpile carpet. He'll probably apply the thermal paste incorrectly too which may cause overheating issues down the line. Hopefully he doesn't ingest any of it and poison himself (The 2006 Mac Pros came with gloves and big scary warnings about the thermal paste their CPUs came with).
He goes back to the store and hands back the old board but tells the salesperson the new board hasn't fixed the issue so now he wants a full refund, despite breaking off a RAM slot clip and a temp sensor connector getting the old board out. He may even have the gall to expect Apple to repair the original fault that he misdiagnosed in the first place. (Trust me, people think like this these days)
After a protracted and labour intensive investigation, it turns out his display was faulty in the first place, not the logic board but the display has now shorted out the new board so he needs another one as well as a display. He might claim the short happened the other way around so now Apple is being asked to supply two $900 boards and a $350 display for $900 assuming he doesn't want some or all of that back as well. So now you see why Apple doesn't supply parts.
For those looking to be outraged about something, you need to ask why these machines aren't built in an even more modular way, and made cheaper and easier to repair. Cars can justify being awkward (a bit) because they get parked on the street and if your engine could be removed with half a dozen large bolts, it would be. Often. But these devices are kept in your home or office. Parts could be designed to be regular shapes, maybe in sealed metal boxes with interconnects and fewer parts per box so each box is only $100-200 to replace instead of one big logic box that has everything in it and costs 75% of the price of the whole computer.
