The middleware that matters has supported Metal for about two years now.

Removing OpenGL isn't far away, as one of WWDC 2019 talks was exactly how to migrate to Metal from OpenGL, while all Apple frameworks now use Metal by default, although some of them still offer the old OpenGL support as fallback.

Middleware might have Metal support but not be used by the game nor tested. And not everyone is going to redo testing and QA on niche backend.

iOS gaming is bigger than desktop Linux....

and smaller than other platforms for the users of such middleware - and due to physical and power differences isn't usually running the same game but a modified one. A lot of porting to Mac depended on OpenGL being an API that you could start debugging on other platforms as well.

Something else worth mentioning is how most major game engines and several game vendors have had native Metal support for some time now.

This mostly benefits new games. If you want to update to (for example) Metal-compatible Unity, that probably means updating to the latest release, and bumping forward from 2017 Unity to 2019 Unity often breaks games. It's a common problem for indie developers who have to upgrade every few years since Apple loves breaking old games.

Maybe something like lutris will emerge on Mac.