Well, most ultrabooks are very difficult to take apart and repair without some serious training.
You can take a PC tower apart and put in aftermarket components yourself for the most part.
But try taking apart a laptop that is about 1 cm thick... I once did that with an HP Folio back in 2012, to upgrade my RAM from 4 to 8 GB, and had a hell of a time putting it back - there's very little room to operate, and when I closed everything up, some cables lost their connections, and I lost the keyboard backlight, and keyboard itself. I spent hours studying the manual (not the one that came in the box with the laptop, but the one that was available on Internet, but aimed at repair shop specialists. And it was not easy to find.
I did manage to fix my laptop on my own, without having to send it to HP's Idaho shop for repairs, and I learned a bit along the way... But the system is usually designed with moronic users in mind - the kind that can take things apart, but don't have the ability to put it back together. Basically, protecting such users from themselves. And those vastly outnumber the people who will actually read the manual, have the right tools, and know what they're doing.
In comparison to Open Source - yeah, I can browse KDE's source code all I want, make my own corrections, compile it on my own system, whatever - and even enjoy and share the results if I know what I'm doing. But if I don't quite know what I'm doing - I'd rather not break my own system, and instead wait for the project to come out with an update.