Yes, it’s by design.
Because, they are not media, they are property values.
That’s actually an important distinction. In Umbraco, media is shared and has no permissions - it’s designed to be centralised for re-use. Content has permissions and there may be property values, including uploaded files, that a user may not have permission to see/use/edit etc.
The files themselves are not really supposed to be navigable on in the filesystem, and this is true for media too - hence the hashed paths.
IIRC these files are supposed to be deleted when removed or content is deleted.