Why would a prospective employer want to remain hidden?

This is very common. Sometimes I will email the listing company explaining that I would no sooner send my resume or any other information blindly to a company that chooses to remain anonymous than they would send me their financials not knowing who I am. Never received a reply. If a company will not identify themselves, I am not going to apply. It might be a company that I would not work for at all. Regardless, that seems to be the way most companies choose to advertise for help.