I think that's because most folks don't know how to do it or aren't able to do so. I don't know how some folks make text posts underneath their URL submissions, unless they are asking mods for help or using some kind of special post type like Ask HN/Show HN. Even then I'm not sure if that's all that is needed for users alone to make these kinds of text+URL submissions without mod intervention.
So if you are saying that this is a thing that is already supported but not used, can you link to such a post that you have made? As it is, I'm not sure that HN even lets you make posts like that.
Otherwise, I guess you can be the change you wish to see, but the guidelines argue directly against the point you're making, basically saying that commentary (which would include a blurb or summary) should go in a comment, not in the submission itself. No one is stopping you from summarizing articles, but just know that AI/LLM and other kinds of generated commentary is against HN guidelines to my reading also, so you would have to do that yourself. It seems likely that what you're asking for would result in more rehashing and less original content, but if that rehashing serves some kind of useful editorial function to raise awareness of content that is otherwise relevant and topical to HN and to its users, then your idea might have some potential merit, but I think it would be gamed by bad actors, and I think the bad actors would benefit more from this change than good actors and existing/future HN users.
I'm cautiously pessimistic on this idea, but willing to hear more about why you think it would help rather than allow charlatans and sophists to invisibly rewrite history and the present to bake in their own ideology and beliefs.