DirectX is a forward-looking standard - Microsoft sits down (or stands up and yells) with developers and graphics manufacturers, and hammers out a spec which a "DirectX X.XX" card must support. Then vendors go and make a card and drivers that support those features. In this way every DirectX 10 or DirectX 11 card can be assumed to support the same things using the same APIs, and if they don't, it's the vendor's fault and they have recourse.
OpenGL, at this point, looks back - Graphics card manufacturers make a graphics card and then shoehorn its features into OpenGL. This way every single card has different supported OpenGL features implemented in different ways.
So sure, "OpenGL" gets some features first via extensions (it's debatable whether or not it's even OpenGL at that point, since the OpenGL standard doesn't even really play into it) - good luck using them, though.
The choice for game developers is pretty easy: support a lot of people (Windows and Xbox 360) using one consistent API, or support a few more people (Linux mostly, with some additional work required for PS3 or Wii) at a huge cost (debugging across vendors, platforms, and consoles).