That's not an inherent limitation of the web or web forums, though - I've seen those features implemented in a lot of them.

And of course, there's nothing stopping anyone from building a web forum and offering an endpoint for third party clients, they just don't, mostly because web forums themselves have more or less been superseded by Facebook, Twitter and Youtube, and no one seems to care about them anymore.

The only pre-web feature that web forums can't really implement is decentralization.