Comment by nsteel

Except they don't use DDR5. LPDDR5 is always soldered. LPDDR5 requires short point-to-point connections to give you good SI at high speeds and low voltages. To get the same with DDR5 DIMMs, you'd have something physically much bigger, with way worse SI, with higher power, and with higher latency. That would be a much worse solution. GDDR is much higher power, the solution would end up bigger. Plus it's useless for system memory so now you need two memory types. LPDDR5 is the only sensible choice.