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Unity and Unreal did the work. Both support Metal.

Both are ridiculously huge and have the money and manpower to do it.

Indie game developers though, which Steam have a sizable amount of, likely do not have the same resources.

Many indie developers do use these platforms instead of writing their own engines. From scratch is usually the exceptions

A large part if not the majority of indie games (both on and outside of Steam) are based on Unity.