Synchronized methods are not allowed to be declared within an interface just because
synchronization is an implementation issue. Since a method is not implemented in an interface there is no reason to put that keyword there.
From
JLS 9.4 Abstract Method Declarations
Note that a method declared in an interface must not be declared strictfp or native or synchronized, or a compile-time error occurs, because those keywords describe implementation properties rather than interface properties. However, a method declared in an interface may be implemented by a method that is declared strictfp or native or synchronized in a class that implements the interface.