Designing for repairability adds some complexity, but it depends on the application. The modularization needed for easy assembly (which helps the factory, particularly when using parts coming from many different suppliers) can also make it easier to repair things. However, at a factory, all the parts are brand-new and quality-controlled from their suppliers, so the process in assembling them is pretty smooth. With repairs, the process isn't so clear-cut: the problem could be different things, there's a diagnosis step involved, and it could be wrong if there's a cascading failure involved (one failure caused another one, and fixing the obvious problem doesn't fix the root cause). Also, doing repairs generally requires more intelligence and training than doing initial assembly; you can automate assembly usually, you can't easily automate repair.

Because of all this, repair ends up being disproportionately expensive. It doesn't help that it's usually not done by the original manufacturer, but instead by others seeking to profit on failed devices.

You seem to be arguing for DIY repair, which I only partially addressed before. I'm not talking about that. Most people cannot repair their own devices: many simply do not have the aptitude for it, and many other have no desire to do so. Repair can be dirty work, or even dangerous, and also require special tools. Replacing an axle on a FWD car, for instance, can be very difficult for backyard mechanics because getting the axle nut off is extremely difficult as it's a large nut and torqued to a high degree, and after years of age and corrosion gets even tighter. I've seen videos of people using 6-foot extension bars to break them loose, and even then failing sometimes, and sometimes they resort to using the car's starter motor to assist, which is just plain dangerous. But a handheld air impact wrench can usually get them off pretty easily. But most DIYers don't have air compressors and impact wrenches like this; they're not that cheap. Aside from difficult axle nuts, car repairs in general can frequently be dirty, and a good way to get your hands cut up. There's nothing wrong with doing this stuff yourself (I do), but to expect for instance some 50 year old woman who has zero mechanical skills and very little arm/hand strength to take on this stuff is silly, and even with more able people, many people these days don't even have a place where they're able or allowed to work on cars (apartment/condo parking lots for instance usually don't allow this).

That's automotive repair, which many people at least are able to grasp intuitively, and the simpler jobs are able to be done with some simple hand tools (wrenches etc.). How about electronics? I'm sorry, you can't expect someone with a background in law (for instance) to be able to use an oscilloscope to diagnose a failing electronic part. People go to engineering school to learn about that kind of thing, it's not something just anyone can be expected to do. Even doing simple electronic repairs with a soldering iron is out-of-reach for most people; I've taught laypeople how to solder, and it's like welding: there's a lot of technique, and not just anyone can master it. It's easy to do a poor job, or to even damage the thing you're repairing.

The point to all this is that we have specialization of labor for a reason (some people are better at certain tasks than others), and that repair specialists exist for a reason: because there's a market for them because many people can't/won't repair things themselves. But by its very nature, repair is not and never will be as efficient as factory assembly. Also, technology improves: repairing a 1965 vehicle might be fun for some, but it'll never have the fuel efficiency, reliability, or safety of a brand-new vehicle (unless maybe you severely modify it, which brings us to a Ship of Theseus argument). At some point, except for a museum relic, it just makes more sense to dispose of the old and buy new. I know I, for one, do not want to be involved in a crash in any pre-2000 vehicle rather than my mid-2010s car. And given how inexpensive many manufactured products are, compared to the labor rates of qualified repairpeople (plus the risk factor of them doing it wrong, intentionally or not), there's a certain point where it just doesn't make economic sense to repair. However, when you're DIYing (or paying a friend with beer), that calculus changes. I will say that YouTube videos and other sources of information on the internet have really helped a lot here too in making repairs more accessible and feasible for many things.