HN generally discourages lightweight chit chat to protect the signal to noise ratio. They more strongly discourage it in these Hiring posts and it seems to me that's the correct way to handle it because this is intended as a free advertising space, not a discussion space.
In part for purposes of making advertisers comfortable posting here so they will post at all, HN more strictly forbids attacks against the advertisers because that would drive away a lot of advertisers and harm the utility of these posts. But, also, these posts have a lot of little ads and it's already a huge pile of info to sort through if you are job hunting, even without chit chat and ugly, long tangents thrown in.
HN likely doesn't have the manpower to lock these threads and institute a pre approval process as the mechanism that protects these posts as 99% (free) ads. So they rely on certain social expectations to make this work.
Since the membership very much values these posts, the membership generally does a good job of respecting certain expectations.
There are spaces on the internet where the meme is "This is why we can't have nice things" and it typically gets stated as a way to say "It's not the fault of incompetent moderation. No, the members are just assholes who won't behave." (Often stated by the mods, who are incompetent and prone to putting out the fire with gasoline.)
HN has both competent moderation and members who more or less try hard to behave. This is why we can have nice things around here, like the Who is Hiring threads, which are a free service benefiting both businesses in need of staff and people in need of employment.