These methods works on the locks and locks are associated with Object and not Threads. Hence, it is in Object class.
The methods wait(), notify() and notifyAll() are not only just methods, these are synchronization utility and used in communication mechanism among threads in Java.
For more detailed explanation, please visit : http://parameshk.blogspot.in/2013/11/why-wait-notify-and-notifyall-methods.html
