Which is weird. RAM is cheap. Isn't it cheaper than going with limited RAM, but expensive/proprietary forms of SSDs? I mean, I get the swappable part, but even a standard USB-connected SSD is going to be way faster than a hard drive. And more RAM couldn't possibly be adding THAT much to the bottom line that this weird setup is needed, can it?
At best I could speculate, and I will, but first: I'm sure you saw the PS5 info. They basically say the same thing Cryo and I were hinting at. They are planning on developers being able to (perhaps with the assistance of a custom hardware block) stream massive quantities of data to RAM. They're each taking slightly different approaches (each with ups and downs) to adding more SSD storage, but overall the storage scenarios closely mirror each other. As far as why they didn't use more RAM, well again this is just speculation, but I can think of a few possible reasons:
First, decent GDDR6 isn't as cheap as standard DDR4. But neither one is really THAT cheap when you look at trimming cost in areas that won't cripple performance. They each know that both companies are building around GDDR6 too. That means supply will get hammered as both companies attempt to crank out millions of machines each using 16GB worth of fairly fast GDDR6.
Second, the memory interface/bus likely makes it challenging to effectively utilize more than 16GB without going well above that mark, which ties into the supply/cost issue. In particular considering this is a semi-custom APU, rather than a pair of chips with independent memory interfaces.
Third, they have to spend their money as wisely as possible to keep costs down. They HAD to go solid state for storage this time around, so it doesn't really cost them that much more to build an aggressive solution with a powerful controller. So if you can achieve similar results by streaming from a high performance SSD to RAM, it starts to make a lot of sense... especially when you're looking at a ~$600 machine.