Collector's combine() never called for parallel stream

Question

I am trying to test running a collector in parallel with the following codes. The "combiner" method is never called, so it looks like the collector is still run sequentially. Why is that so? How do I make it parallel?

import java.util.ArrayList;
import java.util.EnumSet;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

public class TestCollector {

    private static class Staff {

        private String staffFullName;

        private int staffID;

        public String getStaffFullName() {
            return staffFullName;
        }

        public void setStaffFullName(String staffFullName) {
            this.staffFullName = staffFullName;
        }

        public int getStaffID() {
            return staffID;
        }

        public void setStaffID(int staffID) {
            this.staffID = staffID;
        }
    }

    private static class StaffCollector implements Collector<Staff, Queue<Staff>,Staff[]> {

        @Override
        public Supplier<Queue<Staff>> supplier() {
            return ConcurrentLinkedQueue::new;
        }

        @Override
        public BiConsumer<Queue<Staff>, Staff> accumulator() {
            return (list, staff) -> list.add(staff);
        }

        @Override
        public BinaryOperator<Queue<Staff>> combiner() {
            return (left, right) -> {
                System.out.print("combiner called");
                left.addAll(right);
                return left;
            };
        }

        @Override
        public Function<Queue<Staff>, Staff[]> finisher() {
            return staffList -> {
                System.out.println("Number of staff: " + staffList.size());
                Staff[] staffArray = staffList.toArray(new Staff[0]);
                return staffArray;
            };
        }

        @Override
        public Set<Characteristics> characteristics() {
            return EnumSet.of(Characteristics.CONCURRENT, Collector.Characteristics.UNORDERED);
        }
    }
    public static void main(String[] args) throws Exception {
        ArrayList<Staff> staffList = new ArrayList<>();
        for (int i = 0; i < 10000; i++) {
            Staff staff = new Staff();
            staff.setStaffFullName("Staff " + i);
            staff.setStaffID(i);
            staffList.add(staff);
        }
        staffList.parallelStream().collect(new StaffCollector());
    }

}