An outliner needs a database to back it up. Otherwise bullet points or paragraphs cannot be referenced uniquely, and collapsed/zoom state cannot be saved. Much of the outliner power is lost. For PKM you want longevity, so plain text is the core value. Obsidian is built in the assumption that Obsidian might go away someday, and at that time you can still have all your stuff in a recognizable form. We started with desktop apps because (1) they can read local files and (2) once you download something, it will work as long as your operating system allows it to.
If we can somehow magically make plain text work with outlining we’d certainly love that, but right now it’s a trade-off between portability and power.
This thread might help clarify this point a bit: https://twitter.com/zettlr/status/1269347287832109057
Of course, if you do not care about whether your data lives in plain text form, Obsidian is not for you, so I understand why you won’t find it appealing.
Once we reach 1.0 and people can write plugins, there might be developers who make Obsidian into a functional outliner by adding metadata to your plain text, but that’s not something we want to do in the core app.