I think this is because it's one of the few things commonly owned which is expensive enough to customize, but not pressed for space/weight savings. The inside of my PC tower is like 80%+ empty space, totally different than under the hood of a car. No space in there to make it easy to drop in a totally different engine with just a few hookups.

> No space in there to make it easy to drop in a totally different engine with just a few hookups.

It's not about the space, but rather standards and engineering. Old flip phone was as busy around the battery as is modern smartphone. It's hard to change a dying battery if its glued in behind a solid case, no matter the device.

It is about space, laptops don't really have the same luxury as PC.

Currently the trendy ultra small PC cases are going in the same direction with tightly coupled components, not in the connector spec, but dimensional fits.

> It is about space, laptops don't really have the same luxury as PC.

Whats the difference, really? If your laptop CPU and RAM are socketed you can replace/upgrade. If mobos and peripherals were standardized you'd see macs with thinkpad keyboards, and thinkpads with mac internals.