Why we get run time error if we implements Schedulable interface with Queueable interface?

If a public class implements both Schedulable and Queueable interface public class A implements Schedulable, Queueable { public void execute(SchedulableContext c) { // } public void execute(QueueableContext c) { // } } and then this class is enqueued, System.enqueueJob(new A()); a run-time error is displayed Line: 1, Column: 1 System.AsyncException: Queueable cannot be implemented with other system interfaces.