It’s a lot more than just “a few dev hours”. You need to invest in training your testers on Linux, potentially purchasing new hardware, invest in programmers that can deal with writing for Linux, etc… Just because something like BattlEye has a checkbox for Linux support doesn’t mean that all it takes is to click the button and rebuild your game.
It’s not really true anymore. Thanks to the last two generations of console games and all the various re-releases and remakes, AAA game publishers and developers have woken up to the need to preserve their games, their source code and assets. They are hiring archivists as well as having archiving as part of the tasks for finishing off a game.