Well, they are. But the way Cari is currently writing Eternum is that a significant chunk of the story is intrinsically tied to an LI's route, which is different compared to other games where solo routes and harem routes have a branching separate path that players could choose to follow. Here, if you lose one route, you pretty much lock yourself out of certain parts of the story, so going harem is pretty much the optimal path if you want to get the full story experience.
Not to mention Cari is likely to make the harem route canon anyway like what he did in OIAL, so.../shrugs
Which Cari has rectified in this game by making every member of the harem essential to the plot. No Jasmine-type characters here (where an LI really has no story significance save for being related to one of the important characters in the story), no undercooked characters like Aiko or Iris (/grits his teeth and tries to contain his anger upon remembering how much wasted potential Iris was). Everyone gets to take part in the story this time around-
View attachment 2764863
.../sigh Yes, Chang. That also includes you.
View attachment 2764864