Neither RAM nor SSD is embedded in the SoC. The RAM is just soldered on the package. While this makes for slightly shorter path lengths, the only real gain is motherboard footprint.
Well, that and the ability to use LPDDR5 in the first place. There’s no such thing as a pluggable LPDDR5 stick; the design decisions around low voltages & high signaling rates make it physically impractical to survive the longer traces and discontinuities involved in the signal traversing a connector.
You can solder it on the motherboard, you don't have to solder it on the package.
As an old overclocker, I don't believe that this compact soldering is only due to motherboard footprint. It would allow them greatly push the RAM chips to their limits.