Microwave parts do seem readily available, yet it's not been something that has been on your radar. Are you suddenly going to become proficient in microwave repair, or will you simply acknowledge that it's not in the realm of your interest to pursue such things. Similarly, there are many people who don't have the time or care about the user-accessibility of their computer (or their car).
I would guess mainly because a Microwave starts at ~$30 whereas an iMac starts at about $1000 and obviously cars are even more.
This stuff becomes important when you have an item that is expensive enough to bother repairing.
I guess most people would be pissed if they spent $20,000 on a new car and when the clutch failed were told they needed to replace the entire car or spend many thousands of $ having the entire transmission rebuilt rather than being able to take it down to the local repair shop and have them do it within a day for a few hundred $.
That was not the analogy. You can consider "servicing it yourself" and "taking it to a repair shop of your choosing" to be basically equivalent.
If the car has been designed in such a way that a relatively common maintenance task (changing spark plugs, upgrading RAM or whatever) is now much more difficult (thus expensive because the mechanic will charge you for more hours of labour) than it otherwise would have been (or impossible) and reduces your choices of service providers then this is definitely not a good thing to the customer.
Of course sometimes such design changes are inevitable trade-offs against something else. For example a laptop is less maintainable than a desktop but you gain vastly increased portability. In this case (the iMac) it's not obvious what the gain to the customer is.
I don't have to be 'proficient' to pop out a slotted part and put in a new one. If something can be made easy-to-service with next to no trade-off, like RAM, it should be. I haven't had the motivation to look at microwave accessibility, because microwaves are cheap and all do the same thing. But if I was looking at the upgrade option that could make it cook twice as fast as easily as replacing a light bulb, and it was $400 to have the factory install a $50 part, I think I would 'have the time'.
