HN is for thoughtful conversation about topics of intellectual curiosity, and job ads are intrinsically uninteresting. Threads on uninteresting, predictable topics tend to be tedious and nasty (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...). It's easy to predict what threads on job ads would look like: a generic boo-or-yay referendum on each company, repeated each time. But the highest-motivated commenters would be ones with pre-existing grievances, so it wouldn't even be boo-or-yay. It would mostly just be boo. That would make HN worse, not better.

Uninteresting posts like job ads arguably shouldn't be on HN in the first place, but they're there for an out-of-band reason, which is that it's proper for HN to give something back to YC in exchange for funding it. Since that's their only reason for existing, it doesn't make sense to turn them into regular threads.

The opposite is true of Launch HNs, incidentally, which is one of the other concessions HN gives to YC. In those cases, there is something new and interesting to discuss—the launch of a startup—and since each company only gets one such thread, repeats aren't an issue. It's in HN's interest to have open comment threads on those submissions, so we do: https://news.ycombinator.com/launches.