Actually if the company hired a recruiter there may indeed be reasons you can't apply directly and they can't post job ads themselves. I've seen companies make the mistake of accepting absurd NCAs giving recruiting agencies a quasi-exclusive right to applicants. It's not universally common but depending on the kind of contract you enter (and for some of them it's possible to do so informally just by giving them the go-ahead via e-mail) you can tie your hands.