Though technically they do show first-party ads, as pointed out by another comment.

https://news.ycombinator.com/item?id=37889072

Which isn’t that bad. On a smaller scale it’s like a blog author who promotes their own book. It means the ads can be fairly unobtrusive and not mine you for data, while at the same time being highly targeted to a specific audience.

While something the size of Hacker News almost certainly needs a paid admin, something smaller and more focused can still be a worthy community.

I'm not saying that the money is full reason, but at least partial.