Same reason as a) GDDR on dGPUs (I think I read somewhere that GDDR is very much like regular DDR, just with much tighter paths and thus soldered in) and b) Framework Desktop (performance would reportedly halve if RAM were not soldered)
SSD reasons I seem to recall are architectural for security: some parts (controller?) that usually sit on a NVMe SSD are embedded in the SoC next to (or inside?) the secure enclave processor or whatever the equivalent of the T2 thing is in Mx chips, so what you'd swap would be a bank of raw storage chips which don't match the controller.
