It’s because this is not a spin off . All of them can be read as standalones but they are part from a series .
Some author prefer to write standalone single stories while some like to create a universe of characters. A lot of the case what readers call spin off are the author writing and expanding the universe that they plan beforehand, which is the case here. The novel author for this has revealed and shared with fans on social media years ago about her plan on this omegaverse, not only focusing on Miller family but other (Pittman family) as well. She also mentioned that Grayson & Dane are the first characters she conceptualized when first writing omegaverse, so a story written later doesn't really mean it is spin off of the other.
Actually Author zig nim said she created Grayson and Dane’s character first. They are the start of zig’s omegaverse. So they are not side characters or spin off ,but the main characters of Zigs omega verse.
pls continue to enjoy the story! this is a small rant of mine lol - i didn’t realize this grayson miller was the same one that harassed and abused his brother in another story (didnt realize theyre the same author until a few chapters in) and i just dont understand why so many spin off stories are centered around characters that are supposed to be non-redeemable? and the spin off stories tend to be better too ::_:: like what is with making terrible characters, and then giving them a spin off romance with a happy ending?? its all over BL webtoons