This is like responding to someone calling for an open alternative to Facebook by saying we have kubernetes. Linux is a helpful head start to solving the problems needed but it is not the full picture. And depending on what you need to build it might get in the way and require adjustment/fine tuning which is non trivial. There is just so much that goes into an OS in terms of polish beyond just getting some pixels on the screen and responding to input.
Remember that Android/NDK has the same 3D and audio APIs available on regular GNU/Linux, Swift does Vulkan, Playstation has a POSIX like OS based on FreeBSD.
Yet Valve needs to offer Win32/DirectX support via Proton.