Probably because HTML specs were quasi frozen for a long time. As another comment said, XSLT supports it. XHTML conformance would have made the web development

Probably because HTML specs were quasi frozen for a long time. As another comment said, XSLT supports it. XHTML conformance would have made the web development world stricter, and more flexible. Thank you Microsoft for holding the web back. There are also [Server Side Includes](https://en.wikipedia.org/wiki/Server_Side_Includes https://en.wikipedia.org/wiki/Server_Side_Includes) which were used for that. Funny enough, I've used SSIs first and last time around 2020. The project I worked on had a heavily cached landing page (for performance reasons), but needed a way to swap out the homepage banners based on product releases, and the easiest way around that was to output a SSI directive for a separate banners endpoint.