OpenGL has to talk to hardware and is implemented by the hardware vendor. OpenAL does not have multiple implementations, isn't provided by hardware vendors and just wraps the platform audio API.
I'm not sure of your point here. Not sure why multiple implementations is a benefit for users. Audio is generic and hardware is cheap enough that the operating systems just implement and include drivers. It's a cross platform library that meets audio needs, including 3D/spatial audio, much like (and designed like) OpenGL.
