Why must shared objects have a default public constructor?

Vaadin 6 / 7 / 8
1

I am working on an extension for Vaadin 8. My ServerRPC interface declares a method which takes in the following (in the shared package:

public class DomEvent implements Serializable {
  public static DomEvent create(final com.google.gwt.event.dom.client.DomEvent<?> gwtEvent) {
    return new DomEvent(gwtEvent.getNativeEvent().getType());
  }

  private final String type;
  private final long timeStamp;
  private boolean stopPropagation;

  public DomEvent(final String type) {
    this.type = type;
    timeStamp = System.currentTimeMillis();
  }

  public void stopPropagation() {
    stopPropagation = true;
  }

  public String getType() {
    return type;
  }

  public long getTimeStamp() {
    return timeStamp;
  }

  public boolean shouldStopPropagation() {
    return stopPropagation;
  }
}

The idea is for my connector to call the create method. But maybe I misunderstand the required design. This is the compile-time error:

[INFO]
    [ERROR]
 Errors in 'gen/com/vaadin/client/metadata/ConnectorBundleLoaderImpl.java'
[INFO]
       [ERROR]
 Line 236: Rebind result 'com.oliveryasuna.vaadin.common.extendedcontrol.shared.event.DomEvent' has no default (zero argument) constructors

So, I tried implementing a default constructor and noticed that it is this constructor which is called during events. Do I really need to implement something like MouseEventDetailsBuilder? I would prefer the fields that should be final, be final.

Thank you all for your time!

EDIT 1:

I did redesign this class to be like that of MouseEventDetails, being with non-final fields and setters. And I put something like MouseEventDetailsBuilder in the client package. This worked. But I would still like to understand why this must be the design. It goes against immutability.