Godot.

Source 1-2, Unity, Unreal, Gamebryo. Maybe there are more, idk., but my impression is the reason there are no linux games are the devs, not the (externally developed) engines.

Partly devs and their lack of experience (and resistance to having another platform that needs testing on) but also exactly the same mechanisms for every piece of middleware that they might be relying on. The game engine is just one link in the chain.

I would imagine that FOSS engines probably due to having better Linux support for their tools. Godot; Ren'Py; and Panda3D have produced games that run with fewer issues in my experience. That being said aside from some performance issues Unity isn't bad. Unreal seems to be hit and miss. The problem sometiems lies in things being too easy. Devs may have the skill to press the button to make a Linux version but they sometimes lack the will to properly test it.

I'll have to say Ren'py is the easiest, because it creates a Linux build even without the devs knowing it sometimes. You just go to the folder and there are both the .exe and the .sh, and sometimes the dev doesn't even know it, and it works.