I dont know the real technical reason, but I can think of it. I dont think its a good idea to unnecessarily extend/implement classes/interfaces (even though they might be just Marker interface). If they are not marker interfaces then you would end up polluting the class with unnecessary implementations, extensions.
An interface cannot be "unimplemented". If the Object class implemented the Serializable interface, all Java objects would inevitably implement it as well, and subsequently there would not be any need for that interface at all. The way it is now, a class designer can choose whether his class should or should not implement that interface and can therefore decide whether the class will be serializable or not.