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.