Because we didn't want a markup language for hypertext to be the same construct we write blog posts / documentation / etc. in, we wanted consumable human-readable DSLs with no explicitly linked connotations to a browser engine (WYSIWYG renderers, markdown)
The compromise between functionality and convenience since then has been creating frameworks that do these types of post-build steps to give modern functionality (templating, search, accessability options, dynamic theming, reusable custom components, ...) without compromising that idea, while still allowing flexibility on the presentation. Some go too far in either direction and lose the intent, some barely need that flexibility at all and would better be served with pure html.