Index card text is “plain text” so that it can be displayed in multiple parts of the interface (such as the outliner). I think you’re mistaken about hyperlinks being available in the index card text as well. While I don’t in any way speak for Lit & Latt, I can predict that this request won’t be possible to implement due to technical reasons, so you’ll have to look elsewhere for linking up different documents, such as the document notes, where scrivener links do work.