Technically there already is a many-to-many relationship between posts and media items (as far as Featured Images go), but it's not something inherent to posts in general.

The trouble is that while media library entries are technically posts, and do act like objects in certain contexts, they're still largely treated as an analog for a static physical file with some metadata on top. This means that much of the UI is focused on making the file upload process easy and possible on-the-fly, and the metadata is minimized and when possible hidden.

So, if we want to start treating media library entries more like objects with proper recognition of their many-to-many relationships with posts, I feel like there'd need to be a significant re-thinking of how they are treated in the WP Admin. Otherwise it's either going to complicate the existing upload process (which would be bad for average users) or result in a disjointed and unintuitive process.