> We do not own the same copy of that book

Why not? If you hash the book, it’ll match. It’s word for word the same book (editions not withstanding). It’s not the same if you tie in account permissions, of course. Or is this on a philosophical level?

Edit: the downvotes make no sense, aren’t we talking about digital copies here?

Because property law is not defined in terms of hashes. Why would you expect that property law operates on a mathematical construct that was discovered centuries after it?

It's about how copyright law was originally conceived. It did not particularly care about what you did with the information, apart from when you copied it. Copyright is only about the right to make more copies of a work (hence the name!), it doesn't give you any particular rights on its own to what anyone does with any given copy of a work, such as reselling it. This has changed more recently with (first with software) the concept of buying a license to a work as the primary means of purchasing it, which is far more restrictive.

Two copies means two copies. I have a copy, my house burns down, your copy is fine.

Hashes have nothing to do with it.

I can sell my copy of the book to a third person without interfering with your copy. And I can do so without the involvement or permission of the publisher or book store.

If I buy a game, and my friend buys a game, we can both own a copy of the game, without owning the same copy. My disc of Rollercoaster Tycoon 2 is not the same as my friend's disc of Rollercoaster Tycoon 2

If I buy a Billy book case from IKEA, is it not the same as your Billy book case?

Hashing said book case is left as an exercise for the reader.

games are actual physical bits stored somewhere.

It is a distinct physical object. Whether anyone stored digital information about it in a bit-wise manner, does not change that fact of a physical object being unique. The same book can not co-exist at the very same place and location and time in this universe. That makes no sense.