Well, first of all I know multi-platform game exists and in some case it will just work out of the box. If it doesn’t though, not all companies have the money to hire QA for other platforms or devs to look into issues when stuff goes wrong on Linux. Most game companies fail and run out of cash, only the top survives. They don’t have that sort of money laying around to mess around a platform with 2% of users. My previous company certainly loss money on Linux and it was a cause of tension internally.
Secondly, a Minecraft prototype written in c++ and using native OpenGL calls is a terrible example. Even though I understand the dev volunteer his time so money isn’t an issue, it would cost a fortune and take years for your average studio to make a game from scratch like this without a game engine.