Anybody know why the geoserver does not implement a direct sql query when setup a layer?
The developers just hadn't gotten around to it yet. (There is not a huge need for this feature since GeoServer can serve a database view as easily as a table.) However, now GeoServer is able to dynamically alter the SQL used to fetch feature data from databases. This functionality is discussed in the GeoServer users manual at http://docs.geoserver.org/stable/en/user/data/database/sqlview.html