There's a tradeoff involved. Not always, but often: Either [A] the part is easy to access and replace, or [B] the part fits in a smaller space or the hardware can be faster and less likely to fail in the first place.
If you would prefer to trade away smaller, faster, and more robust for accessibility, you should do that. MacBooks are for the folks who side with [B] a lot, and they make that fairly clear.
NB: Of course, sometimes there is a 'tradeoff' where a part of the device is basically just crappier for no real gain, and sometimes the non-standard part's primary gain is 'cheaper', which can be an upside, but it can also be some nickel-and-dime baloney that is clearly a bad tradeoff and the company making the device is trying to get away with it hoping nobody notices. I'm not talking about those tradeoffs.
Examples:
* Using a non-standard screen connector cable because all the available standards do not allow for a single cable to connect device to screen (the case with imacs)
* The effect of soldering the RAM straight onto the motherboard (much smaller and more robust; banging a laptop into a corner is less likely to unseat soldered-on RAM than RAM in a replaceable bay whose engineering is focused more on it being really small than the connection being particularly robust. Yes, YOU can fix it, but your average user perhaps isn't quite as cavalier at opening it all up).
* A single replaceable battery is nice, but if you just glue a bunch of packs in there you can fill every nook and cranny. For what it is worth, apple is still trying to make this as replaceable as possible within reason: I've done a battery replacement on a non-retina macbook air 11, and the 4 separate packs are glued onto a sheet of plastic so you can remove the entire assembly (after disconnecting various ribbons).
Separate from that, there's the notion of 'whose responsibility is it?' – this is a more hairy topic. If you open up your own PC and stick a giant, overclocked CPU in there and then attach a teensy tiny cooler to it all, perhaps due to unfortunate circumstance some of the capacitors on the motherboard blow up due to the excessive heat wafting off of the CPU, which manages to peg itself to 90ºC and survives. If you then make a warranty claim vs. the manufacturer, that feels inappropriate. A manufacturer that tries to stop this from happening would either have to excessively spec out the precise limits of what you can and cannot do (a good idea, but such rules need wiggle room, which means it can't be as fast/small/cutting-edge as it could be), or do what apple does and basically say: Hey, if a qualified apple service engineer isn't doing the work, you're on your own.
I'm not exactly happy with this state of affairs, but I don't see a strictly better alternative either.