Firstly, who is this "everyone" that uses DirectX? Cross platform engines such as Unity and Unreal Engine support both Direct3D and OpenGL.
I think the question is rather why games use Direct3D at all (unless they are Xbox games). It is indeed a valid question why say a game available for Windows, Linux, Android and maybe PS3/4 uses Direct3D on Windows rather than OpenGL. And Windows games, even Windows versions of cross-platform games, actually do almost always run on Direct3D rather than OpenGL.
Secondly, the sites you read that on, did they perhaps have an anti-Windows leaning?
There HAS been a lot of talk about OpenGL being or at least becoming the better choice for game developers in all sorts of places, also on major developers' conferences and from parties that would know, be it AAA developers or hardware manufacturers. For instance check out this post
from Nvidia's official blog.