"Why not?"
I don't know, but I can't think of any developer studio that succeeded that
way.
You think maybe it's because people don't really try, but i doubt it.
"With the same (wrong) argument, you could state that free software will
never penetrate the server market. Wait..."
You're comparing apples with bananas here ;)
The os that got so popular for running on servers was developed over decades
and is a collective effort of countless volunteers.
It's also based on the lucky fact that somebody like rms developed the GNU
system and released it under a free software license. He wrote a free
operating system that was able to compete with other ones available at that
time.
If a second rms would come along and write a free game engine that is as
powerful as modern game engines and make it as easy to make a great game
based on it just as it is to make a great distribution based on GNU, then i
would not doubt that free software would soon penetrate the game market.
But that's very unlikely to happen.