Some wikis purported to exist like this are:
A WikiPageTemplateGenerator? generates a new page from a template. MoinMoin has this ability.
TwikiClone recently added content templates that can be edited through TWiki pages. Formatting templates are still held as non-page files, for now.
But what happens when the template a page is generated from changes ? This could leave many pages made from an old template, which is particularly problematic if you CacheHTML.