I wonder how much of that is because of the asymmetry in time needed to produce quality content, vs. the time needed to consume that content.
I remember when I made "Diary of a Failed Startup" public. It shot to the top of HN, and the top comment was tptackek's "More posts like this, plz." But that one link was a year of accumulated startup lessons learned, experiences, and emotional reactions to things along my startup journey. And it fell off the HN front page in a day and a half. Just by the numbers, posts like that can only make a small portion of total links submitted.
The same thing when I submitted "Write yourself a Scheme in 48 hours". Shot to the top of Reddit and HN, stayed there for maybe a day. Took 3 months to write. Compare that with the volume of material that can be produced by a blogger who spends a couple hours on each post.
When a new social news site becomes popular, people immediately start submitting all the favorite links they remember from years past. But that's selecting the cream of the crop from the last 15 years of posts. Once the community reaches a steady-state, all of those posts have already been read before, and it takes a long time to produce new ones. Instead of becoming a selection of the best articles published over the last 15 years, it becomes a selection of the best articles published in the last day. The latter will naturally have far less quality than the former.
My personal solution has been to care less about consuming content and more about producing it as I've gotten older. This sorta sucks. Producing interesting content is a long, hard slog where you investigate lots of ideas that nobody wants to hear about before finding one that people do. Consuming it gives you the immediate satisfaction of thinking "Hey, I'm smarter than I was fifteen minutes ago." But ultimately, I'd rather be part of the solution than part of the problem.
(And the irony isn't lost among me that this comment is probably part of the problem, being dashed off in ten minutes or so.)