Maybe because, as is the case with procedural generation, the servers need to store this data to ensure that when I jump into a system, my computer doesn't decide it's a 6 planet system with a pristine metallic belt, and your decides it's a 3 planet system with no belts.
That's not how procedural generation works.
At all.
Given the same seed, a pseudorandom number generator will always generate the same sequence of numbers, regardless of what machine it's running out (frankly, what's difficult is getting truly random data out of a computer).
Nothing is stored except the main seed.
Everything is generated on demand, kept in memory while needed, and discarded afterwards.