Synchronization by object

Question

Why syncronization by static variable doesn't work?

public class ThreadSynchronizationTest {
    public static void main(String[] args) throws Exception {
        ExtendsThread extendsThread = new ExtendsThread();
        Thread et1 = new Thread(extendsThread);
        Thread et2 = new Thread(extendsThread);
        et1.start();
        et2.start();
        et1.join();
        et2.join();
        System.out.println(extendsThread.getCount());

    }
}

public class ExtendsThread extends Thread {
    private Integer count = 0;

    public void run() {
        synchronized (count)
        {
            for (int i = 0; i < 500000; i++)
                count++;
        }
    }

    public Integer getCount() {
        return count;
    }
}

I know I can synchronize by (this), or add and sinchronize by private static final Object lock= new Object();, but can't understand why this solution incorrect