Many of those that do also use a bunch of various custom native plugins, middleware etc.
And if we look the most popular/largest games the proportion that use Unreal/Unity gets much lower.
Backwards compatibility is also a thing. If you ship a binary for Windows you can be pretty sure that it will work for 10-20+ years, that’s hardly the case for Linux (because its developers hate proprietary binary software due to irrational “reasons”).
