This is because MSI isn't complete yet - there is functionality not yet migrated to it.
The default stock is hard coded because of that - backwards compatibility.
There are many extensions and even parts of Magento that still rely on "inventory 1.0" and so the default is hard coded so they don't break.
My default stock mysteriously acquires amounts at times, positive and negative.
This in between status is painful but hey, it's only for another year or so ;)
For example for bundles to work, products must be "in stock" in default stock (real stock or "in stock" status with "back order" on, because bundles aren't MSI migrated yet. It's really annoying, but hey, bundles are hard. (the good news are, when you ship it ships properly from your stock)
When MSI is at 100% coverage, we will be able to change who default is.