Not all of them use closed-source non multi-platform libraries.
Unity a closed source game framework was recently ported to linux(just the framework, not the development tools).
There is a partial reimplementation/clone of XNA called MonoGame.
If there isn't a version of a library for linux I guess they have to re-write it. If they are re-writing for mac/linux at the same time they are more likely to use(and benefit from) cross-platform libraries for those ports.
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Not all of them use closed-source non multi-platform libraries.
Unity a closed source game framework was recently ported to linux(just the framework, not the development tools).
There is a partial reimplementation/clone of XNA called MonoGame.
If there isn't a version of a library for linux I guess they have to re-write it. If they are re-writing for mac/linux at the same time they are more likely to use(and benefit from) cross-platform libraries for those ports.