Sadly, The Mac OS platform (Formerly known as OSX) will not be updated as frequently as Linux and NT systems (Windows) as it is not a good practice to commit every new piece of experimental code and then use git actions, act only supports linux / windows.
By the way, it IS available for mac, linux, AND windows, alle executables are in the "Windows 64-Bit" Build. infact even the seperate hidden linux and mac builds are the same.