You know, this is actually an open question in the research psychology literature about furries and has been a question since its establishment in the late 2000's. During the groundbreaking "Furries from A to Z" study Gerbasi Et. Al. did in I think 2007, they found that roughly half of participants at a major furry convention said yes to "feeling 100% human", but also said yes to "wanting to be less than 100% human". The more Therian-like endorsement of "feeling less than 100% human" / "wanting to become less than 100% human" double-hit was in a minority.
That finding suggested an interesting distinction, and I think it's a perfectly good starting point for where "normal furries" and Therians differ. Me personally, I identify with animal bodies but not animal psychology and don't find it to be a spiritual component and rather more of self-actualization. It's an active choice, to me, to openly endorse what I want; my understanding is Therians tend to feel more alignment along that spiritual axis of destiny or trial of the soul, feel free to correct me if I'm wrong.
Way back in 2007, Gerbasi modeled a proposed mechanism for 'species identity disorder' which swiftly did not survive follow-up study or scrutiny. A sociologist, Gerbasi was simply following the template for the DSM-IV-TR's Transgender Identity Disorder, which has since been revised into Gender Dysphoria as of DSM-5. Increasingly, we're coming to an understanding in the Clinical Psychology field that the diverse spiritual beliefs people have aren't reflective of abnormalities in psychology, but rather differences in perspective and experience and how we communicate them.
But in the fandom there is that stigma affiliated with Therianism from the era of toxic rejection of the belief that persists, and you'll see it even in relatively 'normal' people from many backgrounds both in and outside of the fandom. Imagined selves are much more important to those of us with neurodiversity in the first place, so especially for people who are closer to neurotypicality in the fandom, I can see there being the element of Tactile Realism that makes them not consider zoomorphic identity as a goal.
Some people will dismiss things they have eliminated as possibilities for themselves, or simply hadn't considered that the experience is worth the thought. See that a lot in queer fans of mine from conservative backgrounds who are genuinely scared of engaging with their own transgender identity because they "could never make it work", which is a fair fear and I vibe there. It is disappointing when people aren't aligned on the zoomorphism vibe though, so feel you there too. Long ramble, as usual for me.
