Comment by jsdwarf

I'd say in 80% of the cases a pure, static html include is not enough. In a menu include, you want to disable the link to the currently shown page or show a page specific breadcrumb. In a footer include, you may want a dynamic "last updated" timestamp or the current year in the copyright notice. As all these use cases required a server-side scripting language anyway, there was no push behind an html include.