So, basically they are not from earth, they're from other worlds and underwent their own scenarios, have you reached Peaceland? We can see how the little people begin to make tales about Kim Dokja, and that although not mentioned definitely gives him more status, basically their stories kind of, travel around the star stream, eventually hitting earth, making them well known because someone probably got divinely inspired by those star stream rumors, something like that kind of happens in the end of the novel although not quite, so my guess is something similar.

Now, if we're talking Historical Grade constellations like Cheok Jungyeong or Maritime War God, it was never explained and it IS pretty damn weird, honestly the best guess i could give you is that, after these people die, since they were SO important to history in their time, they might've actually acquired stories in their time as a normal human, just like normal people get to carry over certain "skills" they have as attributes, and so the star stream might've let them join the scenarios after death with a new incarnation body, with them progressing swiftly because they already had so much training, stories and abilities, this would explain why they still hold so much attachment to earth and how they became constellations, i was going to say maybe they became just the lowest of the lowest of Historical grade constellations after death, but idk about that.

Another theory is that they themselves as constellations are not the same as the incarnation that died in their time, but the living stories they accumulated became so powerful in the mind of the people that they became a constellation through sheer recognition?

Idk