As a matter of fact, OGL is integrated as DirectX is on Windows. but for some reasons, OGL programmers would use direct coding to OGL with the API. It depends if you want to have special FX from either DX or OGL but also if you're targeting high-end systems or overall. BTW there's many API that impl. OGL or DX FX, and also there are other API like JavaFX or JInput or JAI that doesn't use any external proprietary library to render on.
- OpenGL is not integrated as DirectX on Windows,
using DirectX on Vista for windowed mode.
- There are not many API that implement OpenGL,
there are multiple implementations of the OpenGL API (usually part of the gfx driver)
- There are not many DirectX implementations, there is only one by microsoft
(and maybe in transgaming for linux, don't know) implemented (afaik) on top of the gfx driver
- JavaFX, JInput and JAI are completely different things and may very well use proprietary
libraries to provide their functionality (and JInput afaik does not render anything...)