Actually, that's not exactly true; it's convention to just add the URI because that's the easiest way to do it. But there is support for structured citations, see claim "original language of work" in wikidata:Q43361. There you can see that the reference is "in" a wikidata item, a book, which has its own metadata, and then there are other properties you can annotate the citation with, like page number and quote, etc.
That said, it is easily more complicated than what is currently being done with citoid/VE or citoid/wikitext because of course, it requires you to both a) search wikidata and b) create new items on wikidata which is already more complicated than VE/wikitext which essentially amounts to just inserting text. And it also is a more complicated language, because each reference has to be annotated with potentially specific fields depending on the kind of citation; we have to know which fields go in the wikidata item the reference is "in" (such as volume) and which fields get annotated directly onto the reference (such as page number), and we have to do that with each citoid type (which are just zotero types- see incomplete map between zotero and wikidata types here: wikidata:Wikidata:WikiProject_Source_MetaData/Source_types).
Although I agree with James that it's probably not *immediately* in the pipeline since there is more low-hanging fruit before we get there.