Microsoft is a player on the hardware scene, lest we forget the Xbox or the not so memorable Zune, their dalliance with mobile phones but their tech is nearly always developed by others - so too their software very often but that's another topic in itself...
Microsoft are primarily a software company (clue's in the name!), so it wouldn't be a big deal if they subcontracted Xbox hardware design to another company. Their role is to supply the cash, make strategic decisions, direct marketing - and of course handle the software engineering / server stuff. Having said that, their Surface line does act as something of an exemplar to the Windows ecosystem; their input devices have generally been pretty solid too.
Apple are of course partly a software company, and for me at least, macOS is their crown jewel (if initially brought in with NeXT). But arguably, they are more a hardware company - they don't sell their OS separately, even to Mac users, and their products derive a lot of their appeal from the tight integration of hardware and software. Every product has 'Designed in California by Apple' proudly printed on it.
Going back to their proposed tie-up with Sony in 2001 - if Apple laptops had maintained a distinct identity, even if
wholly designed by Sony behind the scenes, that would be one thing. But if they were generic VAIOs running Mac OS, that would have been a step too far. Sony have their own design language, and ceding product design to Sony would have ripped out Apple's soul. It would have caused a crisis of confidence - why would they then imagine they could design a smartphone? Why not leave that to Sony too?
History has since validated the decision to keep designing their own products; Apple have largely set the pace for the computer / music player / smartphone industry for the past couple of decades (for good and ill), whereas Sony sold off its VAIO business a decade ago, and its Android phones aren't especially prominent. And despite their heritage with the Walkman in the 80's / 90's, their MP3 players were a blip compared to the supremacy of the iPod.
Perhaps Apple would've focused on specialised and higher-end products like Xserve and the Mac Pro and at the consumer electronics level, the iPhone, iPod and iTouch etc? Plus Mac accessories and peripherals.
This would have left a laptop-sized hole in the Mac range, which seems a bit odd given they're the volume seller (Apple since discontinued the Xserve of course, and have all but discontinued the Mac Pro). If they had given up on laptops, they may as well have given up on Mac hardware completely, and just sold Mac OS bundled with specific, validated hardware e.g. Sony VAIO laptops and HP workstations.
To Lexus.
I didn't choose Honda to imply a 'lesser' brand (merely a Japanese one). I just meant that if BMW outsourced car production - their core competence - to another company, in what sense would BMW still meaningfully exist? Lots of car companies use 'badge engineering' e.g. most of the VW Group. But BMW market themselves as something special - the 'Ultimate Driving Machine'. They use rear wheel drive, when most other brands would just save cost / packaging hassles and stick with front wheel drive. But if you take short cuts like that, it erodes the brand - customers aren't complete idiots. It starts with the enthusiasts, but the message eventually gets round.