http://news.ycombinator.com/jobs
It's only the unlaunched ones that don't. Those have to weigh the benefits of launching vs the costs. Usually they launch soon anyway (we encourage startups to launch quickly) and can then give their names in jobs posts. So a post like this is not the last chance a startup will have to hire people; it's just a chance for them to attract a few adventurous ones a month early.
Empirically, they are in fact able to attract good people, which is why they continue to do it. A posting by an unnamed startup may not work on a job board where anyone can post, but here it has worked well for years.