The reason is at the bottom of the article that you link to. The short version is:
-
When the
EmitDefaultValue
is set to false, it is represented in the schema as an annotation specific to Windows Communication Foundation (WCF). There is no interoperable way to represent this information. In particular, the “default” attribute in the schema is not used for this purpose, theminOccurs
attribute is affected only by theIsRequired
setting, and thenillable
attribute is affected only by the type of the data member. -
The actual default value to use is not present in the schema. It is up to the receiving endpoint to appropriately interpret a missing element.