The Java Language specification says that only Throwables can be thrown.
http://java.sun.com/docs/books/jls/third_edition/html/exceptions.html#44043
Every exception is represented by an instance of the class Throwable or one of its subclasses; such an object can be used to carry information from the point at which an exception occurs to the handler that catches it.
The language could have been defined differently, but Java tends to define a base type for any kind of thing that is intended to be used in a particular way and defines methods in that base class to support common patterns. Throwable in particular supports chaining and stack trace printing. It would not be possible to chain exceptions as easily if the only thing you knew about an exception was that it was a reference type.
