I remember in 2007, buying my first MacBook. It came with an enormous 2gb of RAM. I asked about upgrading it. The guy leaned in conspiratorially and told me that Apple’s RAM upgrades were a rip-off, and that I’d be better of buying it elsewhere. So I did, for half of what Apple were asking.
This is a grift that Apple have had for far too long, and there’s a part of me that’s convinced that their move to soldered RAM was to stop people upgrading after the fact more than it was about SOC efficiencies.
A big part of it is that Apple literally places the memory on the same package. It’s literally inside the black package that has the CPU, GPU, and some other dedicated processing units. This system-in-a-package configuration allows the M series chips to have memory bandwidth that basically no other system can match.
Intel tried to put memory on package, but has announced that it won’t be doing that anymore, probably because it’s so expensive to do so.