This happens because the HN submission guidelines at https://news.ycombinator.com/newsguidelines.html say:
“Please submit the original source. If a post reports on something found on another site, submit the latter.”
The main objective is to highlight and share interesting research, and if a post on ‘The Morning Paper’ achieves that and promotes informative discussions like the ones in this thread, then I’m happy.
On a good day, I like to think there’s enough in my posts that the commentary does qualify as an “original source.” On a bad day, the post may be a more straightforward summary of the paper content (though I’ve occasionally tried passing the text of a paper through an auto-summariser for fun, and let’s just say it doesn’t look like I’m going to be out of a job anytime soon!).
For example, the post that presumably inspired this HN thread (the timing certainly suggests that) is more a straightforward summary: https://blog.acolyer.org/2017/12/01/analyzing-software-requi.... The post from the day before on a related topic is interwoven with much more personal commentary: https://blog.acolyer.org/2017/11/30/the-role-of-software-in-....
Even in the ‘straightforward summary’ case, I will typically have spent a couple of hours or more reading the original paper, trying to distil the key points and putting them across succinctly. (It’s not for me to judge whether or not I succeed at that, but this is the aim!).
My hypothesis is that more people have time to read a long-ish slightly informal blog post than they do to scan a 12-20 page research paper. And it would be shame if when presented with only the link to the original paper pdf, they miss out on some of the key ideas within it because it feels like too much effort to read or scan the paper when presented like that. For one paper I’ll be covering next week for example (and this is an outlier), I spent about 5 hours in total on reading, trying to understand, getting the background and context clear, and then writing it up.
In those cases when the original HN link is to the full paper pdf rather than to my post, then I guess an easy way to give readers the benefit of both is simply a comment in the thread pointing to the ‘Morning Paper’ summary... The main thing though, as I said at the top, is that interesting and thought-provoking research reaches a bigger audience.