Because I think if something that big comes into obsidian it has to be a core feature, a whole new kind of object supported natively. There is a lot you can add with plugins and community but what’s in the core shows the drive direction the product is taking and what they will ultimately support and build into the core engine.
Excalidraw is fantastic, but yet it’s a kind of embeddable object that you edit externally, i feel like adding handwritten doodles to notes should be part of the feature set of an editor (just like adding pictures or text) and not provided by a particular plugin. What i mean by that is that I think it should be a core feature expandable by plugins, and not a plugin that won’t benefit from every other plugins nor from the default feature set.
Supporting native obsidian links from inside excalidraw/kanban will always be a bit cluncly, because as much as developpers will have to try to make it look like obsidian content, they can’t rely on default styling, default tools, user plugins… just because whatever content you add there is not inside the default editor. It’s a lot of work just to recreate what can look like you are authoring a regular obisidian content but it’s not.
Perhaps the solution for better content plugins is to have a fully embeddable working note editor so that even in the case of kanban/excalidraw you can view and edit all the notes without leaving that view, and the obisdian editor to be made “embedding-aware” in a sense that plugin could resize it, limit the content of what’s displayed in the note or adapt the font if the editor is getting too small.
The main difference between core functionality and plugin is that core functionality show a commitment to a design decision, and then plugins can further enrich that experience with more nice customization. Whereas a plugin offer a functonality but won’t benefit from any other plugins enrichment beside what you can choose in the plugin options and the developper itself is willing to add. That’s why the big stuff need to be core and part of the plugin API for people to do creative things with them 
There is a good reason why kanban and excalidraw plugin have such a limited syntax support in the link/content they support : they need to re-code all that obisidian logic by hand.