"Why shouldn't it" does not answer the question, which is "what made desktop computers (and servers, to a certain degree) a unique popular product that customers routinely build out of parts".
I am not questioning repairs (which almost never happen, as PC hardware in general is very robust these days) or upgrades to factory-built PCs (which should account for probably 1% of the PC component retail volume). I am wondering why there is an entire industry selling colorful boxes (as opposed to brown cardboard with a part number) with things that are not usable in any way when taken out of the box and are only functional when combined with 10+ other things in somewhat nontrivial way. Forget about "why shouldn't it" and "it was like this forever" and look at this phenomenon with a fresh eye. This is ridiculous (in a factual way, not saying this judgmentally).
I don't know what kind of bubble you live in but it's wild reading this as someone who replaced parts of _literally all the things you listed_.
Just this month:
- Wifes friends laptop: installed new NVME and upgraded with additional 2.5 SSD, had to get SATA cable from china since no one else had it
- Replaced fuel hose on my old e34, need to replace fuel pump on the diesel before end of the year, replaced tires (summer to winter)
- Replaced charging flex cable on my Poco X3 smartphone
- Changed the door gasket on the office fridge
Last month I replaced peltier element in wifes makeup fridge, this summer a starting cap in the office fan, last year old caps in vintage amplifier I got cheap.
My father threw out almost new fridge few years back due to ripped gasket, wife almost got rid of the makeup fridge when the cooling element went out, her friend started looking for new laptop because "old" one had "boot device missing". If you don't care to fix/upgrade or don't know how, then yes, everything is a black box.
"Industry" is f-ing us over and people with your attitude are encouraging it.
I have a couple old radios from the 1940s/1950s. They come apart with a handful of screws and on the inside of the case there is a full schematic. I'd argue that it is perhaps not PCs that have changed but rather the rest of the universe of household appliances.
The first home computers were sold as kits and put together by fervent hobbyists. The original PCs relied on many iterations of standardization and competition amongst clones to become cheap enough to hit peak household adoption. Now PC use is waning in favor of tablets, phones, and smart TVs. As before, the pool of PC users includes a higher-than-average concentration of enthusiasts who enjoy to tinker, thus sustaining a market.
What kind of answer would satisfy you? It seems that you're being dismissive of the very reasonable responses here.
In short, standards exist because IBM built the original PC in 12 months using off-the-shelf parts and published the full technical specs...obviously copycats took off with them and reverse-engineered the bios.
IBM did try to close it when they launched PS/2 with Micro Channel architecture (proprietary, with licensing fees). The industry formed a consortium and created an open alternative, which was bad for IBM.
The colorful boxes exist because there's a profitable consumer market for components, which exists because the standards remained open, which happened because the industry defended them against the company that created the platform. Maybe this clears things up a bit.
I am not questioning repairs (which almost never happen, as PC hardware in general is very robust these days) or upgrades to factory-built PCs (which should account for probably 1% of the PC component retail volume). I am wondering why there is an entire industry selling colorful boxes (as opposed to brown cardboard with a part number) with things that are not usable in any way when taken out of the box and are only functional when combined with 10+ other things in somewhat nontrivial way. Forget about "why shouldn't it" and "it was like this forever" and look at this phenomenon with a fresh eye. This is ridiculous (in a factual way, not saying this judgmentally).