Desktop memory is cheaper, laptop manufacturers are rent seeking with upgrades normally, and with soldered memory they have free reign.
It’s true that often the upgrades are bundled, ie; can’t get 16GiB without an i7. But I think that’s fairly rare at least.
As for 16GiB, there’s a hard limit there for LPDDR3, and intel CPUs do not support LPDDR4 yet.
Oh, and another thing regarding cost, there were some issues a long time ago with the factories producing them, which is why the price is still high, maybe artificially.
https://www.vice.com/en_us/article/wnxb35/the-year-that-the-...
Well, the biggest point of soldered ram has always been to be able to recoup costs by denying people to buy ram from someone else.
Even in the old days it was much cheaper to buy 2 GB of ram and then upgrade to 4 GB than buying it with 4 GB in the first place.
For reference I bought 64GB for my workstation for less than $400 a few years ago. And that's with 25% sales tax.