OpenGL and DirectX aren't rendering engines. No. They are graphics pipeline interfaces which let you submit commands to the graphics card. Such as, "push this bunch of triangles onto the screen using this set of shaders". But you need to come up with the commands yourself. As you've probabled guessed, you can tell OpenGL to draw a million triangles in random locations on the screen, and the GPU will happily do it. But it won't look all that nice, will it? No, not really. It's a computer. It doesn't create stuff, it just executes instructions.
So you need to tell it how to render lighting, shadows, how to draw a realistic sky, how to shade textures properly, how to create a convincing camera, how to generate procedural content that doesn't look like complete crap, how to make tree leaves translucent, how to render fire, how to animate stuff, and so on, and so on, in a way that the graphics card will understand. Depending on how you do this, and on the quality of your assets (textures, and so on - that one is really important because the rest is more or less the same among all renderers) the end result will look more or less pleasing.
As an analogy to the DirectX/OpenGL thing, a computer can add and multiply numbers, but you need to come up with algorithms to actually do useful work.
Now your next question is going to be, "how do I make a rendering engine", the truth is, you won't be able to if you don't know what people want out of a rendering engine. If you are truly dedicated, you can try writing a game directly on top of DirectX and OpenGL, and then look at your finished code, pull out stuff that's got to do with graphics, create a library out of it, and rewrite your game using it, this will give you an idea of how a rendering engine is used. Then write another game, and you'll see there's stuff you need that your library doesn't do yet! So add it in, and try again. And again. When it's sufficiently polished, submit it to the world to see what they think, and iterate until you get something really good.
But keep in mind that the odds of you alone creating the next big thing in rendering engines are close to nil. Engines like CryEngine or Torque3D have - literally - millions of man hours behind them, and have bumped into problems that you cannot even imagine yet (and solved them). So you'd be mostly reinventing the wheel, which is fine as a learning exercise but if you're trying to make money making games, creating your own rendering engine is probably not cost-effective (at least not when starting out).