Why is the Hibernate default generator for PostgreSql "SequenceGenerator", not "IdentityGenerator"?

Maybe because afterInsert generators are generally broken for PG in NHibernate because it uses OracleStyle out-parameter style which is not supported by npgsql-ADONET driver which returns the result as query result and not out parameter.

SQL: INSERT INTO .... returning id into nhoutparameter; :nhoutparameter = null;

using Oracle this works

command.Execute();
object id =  command.Parameter["nhoutparameter"].Value;
Assert.NotNull(id);

in PG not. It should be

object id = command.ExecuteScalar();