For one thing, HTML was originally designed for read-only publishing. User interaction was gradually added over time.
For another, if you expand the scope of HTML to entire widgets, it'll become an incredibly bloated language. At a certain point, it needs to be building blocks rather than a pre-built solution.
Why don't Legos come with cars and boats already built in the box? Because some people don't want a car or a boat, they want a plane.
You might get some pieces that lend themselves to a particular purpose (select box, blockquote, etc.), but the language is not highly opinionated about what you do with them.