Linux doesn't know how to compute how many frames were generated in a second
Nah, most user-oriented distros definitely have the necessary API for this
Aren't 3D games scripted in a high level language that can be compiled for any machine?
Not always (some optimization might require OS/platform specific assembly code). And high-level language in itself doesn't mean the necessary libraries for games are available for all platforms.
Can't the game engine output OpenGL instead of Directx?
In theory most engines can be written to do that. In practice it's possible they didn't bother, graphic is just one of (admitedlly large part) of the equation, there's also anti-cheat (which rarely works on other platforms), netcode (can be way more complicated if the original design targets a specific platform) etc.
Doesn't NVIDIA Write OpenGL drivers?
They do, but whether they have the equivalent feature & performance of their Windows counterpart, and compatible with the myriad of distro configurations are coin toss
Is OpenGL not powerful enough?
OpenGL is just a spec. It's the driver and engine implementation that varies. It can be expected that devs and companies will spend more effort to platforms with more users.
Is it not being updated?
It's regularly updated, but again, the driver and engine compliance with the newer standard varies.
Is the Linux Gaming community not big enough?
Not large enough to make most devs prioritize it.
Do game developers use Windows API calls?
They usually do, either indirectly from the libraries/framework they use, or directly.
What's the big deal with 3D games on Linux?
Supporting a platform requires non-zero effort, and unless it's obvious that not doing it loses more money than the cost of the developer time, it will stay that way.