You raised a good point about rocket league, which seems to be using the ancient unreal engine 3. Epic basically updated ue3 to support directx11 but neglect updating vulkan/metal support on the old engine. But Fortnite is using unreal engine 5 though, which has excellent Linux support. Epic had a presentation bragging about how they got Fortnite running on Vulkan as "same game, not port", so the decision to not support Linux is certainly not a technical one.