I really wonder why the PC is a different story. What is it that makes it special in this regard?
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.
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.
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.