Why can't you use primitive types in generic classes/methods?

Question

Java

Hello.
We have this code:

private static <T> T testAutoboxing(T obj)
{
    return obj;
}

public static void main(String[] argv)
{
    System.out.println(Learn.<int>testAutoboxing(5));
}

During type erasure, int becomes Object, and the value 5 can be equated to Object using autoboxing.
The question itself is: why is it forbidden to use primitive types in this case, if they have autoboxing, and why does it not work?
PS I read the documentation, I read the book and I know how reference types differ from primitives. The question may be elementary, but I'm a beginner with a week of experience. It is only unclear why, having declared a primitive, the compiler does not convert it into a shell, and there you can push it into Object, as happens with other reference types. Or is there a fundamental separation of primitives from objects of any class?

Answer 1

The book only confronts the fact that only reference types can be specified. If you want primitive, then you need to resort to encapsulation.

Answer 2

During type erasure, int will become Object,

No. Where did you get it from? For primitives, types are not erased.