Why is the more interesting, deep, and unique character the one that gets you definitely disgusted?
First off, Astral Chain is a first party Nintendo title. They own the IP and have published the game everywhere, they just partnered with Platinum to develop it. Second, the Spirits have been an absolute mess with regard to cross promotion, so they don't tell us anything. New Super Mario Bros. U Deluxe (God I hate that name) didn't get spirits until months after release, Link's Awakening got Spirits months before it released, and now Daemon X Machina of all things is getting Spirits despite actually being a Marvelous product... Super Mario Maker 2, Fire Emblem Three Houses, and Astral Chain are all first party titles without spirits in the game as of right now, and I sincerely doubt we're getting playable reps from all those games. Super Mario Maker 2 especially has no real reason to not have Spirits, but also wouldn't have Spirits that would fit super well with a character unless you just absolutely shoehorned them in with like Waluigi. Also, like I said, Spirits have come at weird intervals, so we may see Spirits for these games show up at a later date... or they might just not make Spirits for these games because they didn't feel the need to cross promote with them as much. It's hardly a smoking gun that Three Houses doesn't currently have Spirits in Ultimate.