The problem with keeping your notes in the project namespace is that you would still need a place to organise your notes and remind you where they are. If sandboxes were linked to articles, then you might have a dozen or so different sandboxes linked to a single article space - several of them forgotten (because you don't have a user pages to organise your sandboxes). I have a whole pile of abandoned article ideas in my user space. That doesn't mean that they will never be used, just that my attention got consumed by other (usually less productive) things. Granted, you could keep track of these things by looking through all the pages on your watchlist...but that would be far less productive (given that many people have watchlists with thousands of items.