Why is Stream.sorted not type-safe in Java 8?

This is from the Stream interface from Oracle's implementation of JDK 8:

public interface Stream<T> extends BaseStream<T, Stream<T>> {
    Stream<T> sorted();
} 

and it is very easy to blow this up at run time and no warning will be generated at compile time. Here is an example:

class Foo {
    public static void main(String[] args) {
        Arrays.asList(new Foo(), new Foo()).stream().sorted().forEach(f -> {});
    }
}

which will compile just fine but will throw an exception at run time:

Exception in thread "main" java.lang.ClassCastException: Foo cannot be cast to java.lang.Comparable

What could be the reason that the sorted method was not defined where the compiler could actually catch such problems? Maybe I am wrong but isn't it this simple:

interface Stream<T> {
    <C extends Comparable<T>> void sorted(C c);
}

?

Obviously the guys implementing this (who are light years ahead of me as far as programming and engineering is considered) must have a very good reason that I am unable to see, but what is that reason?

Copyright License：
Author:「Koray Tugay」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/53219523/why-is-stream-sorted-not-type-safe-in-java-8