I think all of the answers so far forget what it was like with Java 1.0. In Java 1.0, you could not make an anonymous class, so if you just wanted an object for some purpose (synchronization or a null placeholder) you would have to go declare a class for that purpose, and then a whole bunch of code would have these extra classes for this purpose. Much more straight forward to just allow direct instantiation of Object.
Sure, if you were designing Java today you might say that everyone should do:
Object NULL_OBJECT = new Object(){};
But that was not an option in 1.0.
