Apple decided they want memory modules that only contain memory because the storage controller is on their own silicon and they have direct low level access to the storage chips if they do it themselves. Probably cheaper (for them) without redundant controller parts they don’t need too.

Surprised nobody has reverse engineered this yet to make compatible upgrade modules. Probably not enough money in it.

I just got mine with 1TB. I added a second external SSD for a bit more space. Good enough for me. The only issue is that I can’t point the Dropbox app at the external drive. The new one has to point to the user home folder (guess I could move all of that to the external drive).