I mean, you can argue x causes y for a lot of comorbidities in psychiatry and you may not be wrong. I don't think it limits the utility of the diagnosis. I also have a hunch the poor awareness and understanding of dissociative disorders and dissociation itself really is probably a strong reason it seems like it is only existing as a comorbid type of psychopathology to others like depressive episodes. Someone suffering from chronic dissociation as the main and only substantial issue may just never present to psychiatric care due to the still somewhat vague nature of how dissociation is described and how recent of an entry the word is in casual use. People can piece together more what anxiety or depression are and are more likely to end up in treatment for them because its often routinely screened for, they're not recent entries into common use in the english language so people have more of a name to put to their feelings, and they've been extensively defined over and over due to their prominence.

On the flipside, something like dissociation is probably going unnamed in many suffering from it even to the point of having psychosocial impairment to some degree, and in other cases is probably, due to difficulty in naming and describing it especially amongst middle aged and elderly segments of the population for whom its a pretty foreign concept. I think more bipolar individuals would get a hunch regarding manic episodes if those were, similarly but less so, something the general public knew how to describe. If mania didn't commonly co-occur with depressive episodes as in BD then I would wager most people with some degree of mania would never even present to psychiatrists for diagnosis for this reason, and I think that may be the case with dissociative conditions.