I am well aware of the attempts to replicate expensive exotic cars or even build tuner cars from kits (such as the ultima gtr720) that purely built for handeling and speed. Most of which are not street legal in the united states and have little practicality in the real world. Given that these are so different than luxury cars such as BMW, mercs, audi, etc., I still do not think the car example relates. On a side note, I build go karts for fun. They are definitely not street legal, the interior doesn't look anything like a 750LI, but damn it can drift around a turn. Not so sure I would be willing to do that in a $100k bmw haha. But hey if my go kart was street legal, I would drive it everywhere!
Anyhow, I build computers for fellow engineers along with people doing 3D rendering, audio engineering, video editing, etc. I guarantee you that computers are not just for the gamers and those who feel like giving building a try. In the custom computer world, home built computers utilizing raid controllers to control the flow of data to SSD's and overclocked processors have overpowered the 5k+ custom ordered computers from dell (or hp i cant remember). When professionals request a custom build, they are set up in such a way that factories cannot meet their expectations or would charge insane amounts to do so. That is the driving force behind why I build computers for others (i dont charge for my work either because I enjoy it too much 
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I actually know quite a few engineers who work at intel (a major intel facility is just 20 minutes from my house) and are very knowledgeable about the 14nm integrated processor architecture. This is the way of the future, but is not going to cause custom build computers to cease to exist. Intel makes far too much money from the custom pc world to risk that. But it also will have a profound effect on advancing major computer companies (dell,hp,etc)
I realize 95% of the computer market is just looking for a computer to turn on an preform basic tasks. I just find it astounding when they spend twice as much for an iMac than a custom build that I recently did for a client that has 2 to 3 times the processing capabilities. (not just the processor alone - this takes into account overclocking on the processor, ram, having an ssd raid array, motherboard to handle the sensitive current and high data flow, etc.) In the end, all that matters is how many floating-point operations per second is achieved.
But hey, if owning that beautiful aluminum computer is what makes you happy, more power to you 
People dont build their own cars because it requires skill, time and interest that they neither not have or are not interested in. The only difference between a custom car and a lux car like say a Pagani is that a car requires to pass govt regs to be sold legally while custom cars can be built but not operated in public roads.
I've seen custom cars fitted with pretty good interiors made from scratch. You just need to find the people with the parts or craftsmanship to do it properly. Again, some people enjoy doing such things while others rather fork out the money for something nice.
The software on the Mac is different than those on the PC. Hardware design is also different. End user experience is also different.
These are parallels of Apple products to luxury cars that I named earlier. Apple also follows the customer experience model of the Ritz-Carlton. Why else would people pay "double" for a Mac that does the same thing with a prettier case? Does Apple have to slather their products in leather and chrome to make the connection ever stronger?
People can go to the grocer and make themselves a pretty good meal for 1/10th the price of going to a 5 Michelin Star resto but people still get wait listed months in advance to get a table. Call them crazy for wanting to eat in a fancy place rather than the comfort and privacy of your own home.
There's a saying, you can fool some people some of the time but you cannot fool all people all the time.
Of course Apple and to some extent Dell et al cannot cater to the whole market like custom jobs as these are not economical or even profitable for them to do so. You did mention not charging anything for your labors.
Intel and other parts makers see this also as demonstrated by the declining sales of desktops.
The market will dictate when custom computers will cease to exist in the same way as Linux on the desktop/notebook has been a running meme since the 90s.
Remember, this is Canon Rumor forum and not Anandtech or HardOCP. We just want our pretty pictures (and maybe video) processed with little fuss.
Not putting down custom jobs or commodity PCs. I've seen my brother's gaming setup and it's insane for what he can do but for me I rather spend the extra as I am looking for an appliance more than a project within a project.
