Most of the work in getting a game to run in Linux if you're not using a wrapper is to get the engine to work with OpenGL.
Now, two of the largest graphics engines in the industry is supporting OpenGL/Linux, and you have plenty of devs with in house engines that support OpenGL.
Strangely, the only ones complaining about how hard it is to make Linux ports or natives are big companies. One can wonder why.
If small companies can do it, if a few people can do it, why not a big company?
I hope it isn't a question of competence, and seeing companies like EA and Ubisoft, which I consider to be immoral I wouldn't be surprised if they are paid by Microsoft to not make their games available on Linux.