Historical reasons, I guess.
Petr had already started developing the book module before Gustav proposed to split resources and activities
Since then all the different kind of resources were rewritten as resource types, but it was not possible to fit all of the bookmodule's data inside the database field reserved for every resource type.
From a user's perspective the bookmodule definitely belongs with the resources (and I believe in between all of the security stuff Petr is rewriting it to become a multipage resource type).
But looking at the code you can see it has a lot more in common wih the other activity modules, and I guess that's why it is still there with the other activities.
Sounds like the concept of splitting of resources and activities should be revisited. For the same reason that the original split was a good idea, this is now confusing to new teachers. I feel silly when I explain to them that all static resources are in one popup except for Book which is under Activities. All the technical issues not withstanding, the user interface is for students and teachers not programmers.