I am pretty sure it is a consequence of consumer preference. I can see it from my own behaviour - I am a power user of all things computing and it has been decades since I upgraded a harddisk.

Regarding storage, part of the advantage of their soldered storage is higher access speeds. Combined with the fact that, as you note, people rarely need to upgrade their storage, might as well use the faster storage. This is probably particularly a benefit for the 8 GB machines, which I assume use swap regularly.