Orbital and SpaceX could easily take their craft in for docking themselves, but NASA's rules require them to do it this way. NASA's rules are that nobody is allowed to put something on a trajectory that intercepts the ISS, even for an instant, for any reason. This is the reason that a secondary payload on an earlier Falcon launch wasn't allowed to be put into its desired orbit. An engine failure on the Falcon's first stage required it to take a modified trajectory into orbit, at which point boosting the secondary payload would have required that, for an instant during its boost, its trajectory pass through the ISS. For this to be dangerous, it would have required the engine to fail in the middle of its burn at a very precise instant. NASA disallowed it, so the secondary payload wasn't able to perform its mission.
So, bringing a spacecraft in for docking requires you to put your craft on a collision course. Docking is just a low-speed collision, after all. NASA will not allow this, so anyone bringing payload to the station has to rendezvous and place the craft within range for the ISS to grab it and bring it in.