It’s clearly explained in the FAQ:

> How do I make a link in a text submission?

> You can't. This is to prevent people from submitting a link with their comments in a privileged position at the top of the page. If you want to submit a link with comments, just submit it, then add a regular comment.

If you want to add more context, add it in a comment. If it’s good people will upvote it.

One reason I could think of is because it encourages informative titles - I enjoy being able to click directly from home page to article, without having to look at the comments. A lower percentage of posters would explain Ruff in the title if there was a description. (Clearly it's currently not 100% either, that's why I'm here.)

Though this is something that keeps coming up on Reddit as well, I assume there's a reason why HN went with this design.