Lets be honest here most games have the logic and rendering elements well separated and distinct from the OS platform code. I am going to assume that the difference in the code base between the platforms is probably less than 1-2%.
They could support Linux quite easily if they wished. The proof is quite literally in the fact that most of the AAA companies are falling over themselves to port their engines to mobile.
It is worth mentioning that because of Steam, Desura, Kickstarter, and the humble bundle that Linux is growing a consumer base wanting commercial games. Once that base gets large enough of course companies are going to want to support it.