My initial confusion came from looking at Block Hub [1] which appears to be a collection of UI components. But the underlying Block Protocol specification [2] is all about structuring data around entity types and entity identifiers. Looking at a specific example of a block [3] the UI component is displayed next to the underlying data and schema.
My guess is the creators of Block Protocol want to promote the free exchange of structured data and decided that coupling the data to UI components is the best way to do that? It’s notable that the web has faced challenges encouraging use of both structured data (Semantic Web) and UI components (Web Components). Maybe they can be more appealing bundled together.
I do hope the Block Protocol project tries as much as possible to reuse existing standards for data and components, building upon whatever we have that already works well.
[1]: https://blockprotocol.org/hub