A quick answer might be that wait() and notify() are used to control behaviour of a particular object, which interacts with more than one thread.
A Thread does, of course, have these methods, as it inherits from Object. However, it is rarely, if ever, useful to use wait() and notify() on a Thread.
Look at some typical examples of how wait() and notify() are used, and then think how you'd implement them if wait() and notify() were methods on Thread not Object. Chances are, it would be harder or impossible to achieve the same functionality.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.