Unfortunately, there are no plans to bring Planet Coaster to macOS. That is not because Frontier doesn't want to, but because Planet Coaster requires DirectX in order to run.
DirectX doesn't work on macOS. The latest version of the competing variant of DirectX on macOS, named OpenGL, is not being supported by Apple at this time. Because of this, Planet Coaster cannot be developed for macOS.
There is a solution that works for owners of Mac computers that want to play Planet Coaster; use BootCamp. This works for a lot of players, but do know that it is not officially supported by Frontier.