Earlier quoted context omitted.
Because it's a stupid view of the world. What if you bought a BMW and they said "sorry, if you want to replace those tyres or wiper blades then you need to bring it in to BMW, even though we don't actually make any of thee parts ourselves". (Yes, I understand these are serviceable parts, replace above with car battery if you like...)
I'm not sure how you meant to map this onto a situation of replacing the motherboard because a RAM chip died. You don't have to replace the whole engine if your car battery dies, even in a BMW, regardless of warranty status.
Yes, that was my whole point... I don't have to replace the engine because the battery isn't glued to it. So I may have to remove a few nuts and bolts and it may not be easily accessible, but they are replaceable.
If someone told me that I should replace my car because the warranty had run out, and besides there is now a more powerful version out I'd probably call them mad!