In an old comment that I can't find, dang explained that the problem of enabling the comment section in the job posts is repetition. Jobs post of each company are very similar, and they may even be equal if theay are hiring a second person or the first try didn't get any candidate. So everyone that likes the company will repost the same comment, and everyine that dislije the compay will repost the same comnent, and the result will be the same discussion again and again each time.
If you are interested in what they are doing, I sugest to look at their blog and pick the most interesting article (in your opinion), and post it to create a new discussion. (You can even post the first commemt to sugest the topic you want to discuss.)
This seems to be asked once a month. I am surprised there isn't a spam filter explicity for it at this point:
> Is there a specific reason such a feature doesn't exist
Maybe no one has, or not many people have, asked for it? And I'm not sure what, if any resources YC would be willing to devote to adding something like that.
YC probably prefers adding a bit of friction for people interested, instead of having the risk of negative comments about the companies under the job posting.
Those job posts are from companies that YC has invested in. Most of those companies are yet another way of trying to get rich without any real product of worth. If people were allowed to comment on them, the reality of those companies will get exposed. So such posts are exempt from the glorified rule that let the HN community decide what's interesting and what's not and comment and vote accordingly.
Those threads would probably get filled with negativity. (Why does this company even exist? I could do this in a weekend!) It would also create a conflict of interest for the moderator (dang); if he kills posts trashing YC companies then it's censorship but if he allows them it's like throwing startups directly into an orange crab bucket.