"Open standard" is not the same as "open source".
I don't think OpenGL works like that though: the "open" in the name was mostly just a marketing response to M$' quite closed DirectX & Direct3D product. DirectX/Direct3D ran only on Windows. SGI made it's competing 3D pipeline that it called GL available on any platform from UNIX to MacOS to Windows, so SGI rebranded GL as OpenGL. The point of the "Open" in the name was that SGI GL was being released in such a way that it could be implemented on any hardware and/or OS that someone wanted to use.
There is no fee to acquire the OpenGL spec and implementation guidelines. Anyone who wants to can just download them and start working. I downloaded them in circa 1998, read them, and decided I had better things to do with the next decade of my life than implement OpenGL.
The reason MESA has always lagged behind commercial implementations is IMHO because an OpenGL implementation (1) is very complex; (2) is not really well suited for a CPU with only a few cores; (3) is certainly more complex to implement on a MIMD CPU than on a SIMD GPU.