Earlier quoted context omitted.
For higher power parts, heat dissipation is the primary reason. Most of the high end CPUs are purely heat limited in terms of performance. A TDP of 90W is about the limit for CPUs using cheap cooling techniques, and adding ram would lower that by 10-30W.
If you interleaved the memory with the processing, wouldn’t it decrease the heat density, though, which afaik is the limiting factor? You can get rid of quite a lot more heat than that if it’s not in too small an area to conduct away quickly.
Processors also tend to run hot, while RAM is more reliable when cool. Much like some parts in human biology benefit from being outside of the core (enough to make it an evolutionary benefit even when it's quite an obvious defect due to vulnerability), having RAM in it's own cooling sub-domain makes sense, and since it's already out there the blade interconnect card design is space, cost, and cooling effective.