I can fully relate to the frustration about media management in WordPress.

Why can't an image be attached to multiple posts? Because there are functions that return the ID of the "parent" post/page, so they would have to be altered.
However, it is not like something like this hasn't been done before. A post can have multiple tags and categories, but an image can't belong to multiple posts? Pffff...

Thankfully there is this plugin, which comes in handy from time to time, though it doesn't really solve the big problem: