Metal tessellation is just such a different API that porting a DX11 engine's tessellation over to it is pretty daunting. If I were a game developer, my response would almost certainly be to just disable tessellation for Metal (and thus suffer an all-too-common massive loss in visual quality in the macOS version). For games where tessellation cannot be disabled, then I probably would just not release that game on macOS. If Metal had the same 5-stage graphics pipeline as DX11, with hull/domain/geometry shaders in addition to the vertex/pixel shaders, then Metal tessellation would be much more interested on the desktop side of things. However, their iOS GPU doesn't work that way, so Apple is never going to expose a DX11-style pipeline.