Because you cannot embed a page on a page.
A NetWebPage is a "page" - in other words it has a header, footer and so on in html. html pages only have one HEAD section etc.
A NetWebSource is _not_ a page, it is designed to be embedded on other pages. It does not have the HTML head part, so cannot be a page on it's own.
A NetWebBrowse and NetWebForm are special. They can behave either as "includes" on another page, or if called directly will be wrapped inside a "generic page".
The Header and Footer tags you specify on the WebServer procedure have to be NetWebSource, and will be included before, and after, Browse and Form procedures, ONLY IF those browses & forms are called as "pages".
Cheers
Bruce