Ask PG: Why does HN keep stories with no votes/comments for 2+ years?

I used HNSearch just now to find a recent link about African charity "success" [0] and I was surprised that, by the second page of results, there were stories from 2+ years ago that have no votes or comments. I would've guessed that you guys culled those after a time - "If no activity after x years to a link, mark it dead" or something like that. Examples:

https://news.ycombinator.com/item?id=2345661 https://news.ycombinator.com/item?id=4093929

The fourth page of results is almost all "older than one year but no activity" pages: https://www.hnsearch.com/search#request/submissions&q=africa...

My original search for "africa" in stories: https://www.hnsearch.com/search#request/submissions&q=africa...

Just curious - I see how often [dead] is used here and I suppose I was a bit surprised to see those older, no-activity results show up.

[0] It was this one: https://news.ycombinator.com/item?id=6832148