Not everyone is on HN for years. Some people may not know that those are job advertisements.
I agree. Once per week, someone ask why it is not possible to upvote or comment in one weird post or if HN has ads, sometimes with a screenshot in case people don't believe it.
I think only Y Combinator companies can post "Is Hiring" ads. Everyone else posts job ads on the once a month "Ask HN: Who's Hiring" list.