Because packages aren't accessible. What the word "Accessible" in the name
AccessibleObject means is effectively has an access specification and associated scope. For example, an object might have
public access (at which point its scope is irrelevant), or
private access from within
class Foobar, or package-local (default) access in package
AccessibleObject also provides access to annotations that are declared on a Java source element, but this isn't its primary purpose.
Packages don't have this. All packages in Java are public, and are globally accessible. (I believe Java 9 introduces a system that is going to change this in some ways, but I believe it just introduces a new orthogonal system of classification that isn't going to affect AccessibleObject and packages).
Packages can have annotations, but access to these is provide through alternative means, so have no need to extend
AccessibleObject to provide the relevant API for this.
This means that the methods in
setAccessible(boolean)) are irrelevant for packages, because packages are always accessible and this is not something that can be set. Therefore, extending this type would not be appropriate for them.