Why do so many job postings on HN not mention the company name?

A lot of these companies are extremely early stage and probably wouldn't have that much information online for you to find by researching, even if you did know the name. The best way for you to find out about them would be to contact them personally.

Those startups probably haven't announced that they've taken (YC) funding yet. That sort of announcement is usually a good way to get some press.

Hmmm among the responses this seems like the most likely explanation. Thanks!

Privacy, most likely. Knowledge is power. Don't let your competitors know what's coming until it smacks them in the face and all that.